Ichikokubashi Bridge is the May entry in a twelve-bridge sequence that connects a calendar associated with Mutsuo Koshikawa to a handwritten list found in Kyoichiro Kaga’s mother’s apartment. The bridge matters less as a demonstrated crime scene than as a recurring clue: its name appears in both records, and Kaga uses the calendar’s monthly arrangement to propose that Koshikawa may have intended to visit it in May. The evidence does not establish the date or purpose of such a visit.

Setting and Place in the Sequence

Ichikokubashi Bridge belongs to a sequence of bridges in the Nihonbashi area, one assigned to each month of the year. It follows Tokiwabashi Bridge in April and precedes Nishigashibashi Bridge in June. In the complete order, Yanagibashi Bridge is January’s entry, Asakusabashi Bridge February’s, Saemonbashi Bridge March’s, Tokiwabashi Bridge April’s, Ichikokubashi Bridge May’s, Nishigashibashi Bridge June’s, Nihonbashi Bridge July’s, Edobashi Bridge August’s, Yoroibashi Bridge September’s, Kayababashi Bridge October’s, Minatobashi Bridge November’s, and Toyomibashi Bridge December’s.

This ordering is reproduced in two forms. Searchers find bridge names annotated on every page of Koshikawa’s calendar, while Kaga possesses a separate sheet listing the same twelve bridges alongside the months. The duplication makes Ichikokubashi Bridge part of a patterned record rather than an isolated place-name. The available evidence associates it with a proposed journey, but does not document a visit to the bridge itself.

The Calendar and the May Reminder

Kaga’s interpretation begins with the timing of Koshikawa’s annotations. Koshikawa was murdered in March, yet the calendar already carried bridge names for April and the remaining months of the year. Kaga reasons that Koshikawa probably wrote the names onto the calendar when he first obtained it, suggesting that the sequence had enduring importance to him. In this reading, the calendar does more than record months: each page may renew a reminder of a particular bridge.

Kaga illustrates the idea through the change from April to May. When Koshikawa turned the calendar page, the name Ichikokubashi Bridge would have appeared as the next month’s entry. Kaga proposes that he might then have read it as a prompt to go there during May. This is a reconstruction offered as plausible, not a confirmed account of Koshikawa’s thoughts or movements. The May entry gives the hypothesis its structure; it does not prove that the visit occurred.

The question of when such a visit might have taken place remains open. Kaga’s instinct is that it could have been May 5, Children’s Day, because it was a public holiday. He also considers Constitution Memorial Day on May 3, but finds no link between either holiday and Ichikokubashi Bridge. The distinction is important: the calendar supports a month-by-month pattern, while the proposed holiday dates remain speculation without an established connection to the bridge.

The Matching Handwritten List

The bridge’s significance expands when Matsumiya sees the note Kaga found in his mother’s apartment. The note contains the same twelve month-and-bridge pairings as Koshikawa’s calendar, including Ichikokubashi Bridge for May. Matsumiya is dumbfounded by the exact correspondence, and Kaga is similarly astonished. The match connects two otherwise separate objects and turns the calendar’s annotations into part of a wider investigation.

Kaga is careful about what the note can establish. He says that it was found among his mother’s effects, but its handwriting was clearly not hers, so its original owner is uncertain. He suspects that Shunichi Watabe wrote it. Kaga tells his superiors that his mother had a long-term relationship with Watabe, yet admits that he knows almost nothing about the man and has been unable to work out the note’s meaning. He asks the task force to arrange handwriting analysis as a personal favor. Thus Ichikokubashi Bridge is present not only in Koshikawa’s calendar but also in a document whose authorship and purpose are unresolved.

Investigative Role and Limits

The calendar’s annotations initially puzzle the search team. Their members consider whether the bridges correspond to events Koshikawa attended, but their inquiries uncover nothing that verifies this explanation. Kaga instead focuses on how the entries might have functioned as reminders, using the monthly progression to identify Ichikokubashi Bridge as the next anticipated destination after April. The contrast between the unsuccessful search for public events and Kaga’s attention to the page-turning sequence marks a shift in how the clue is read.

The list also becomes a point of contact between the investigation and Hiromi Kadokura. When Kaga shows her the sequence, she recognizes that Sakagami has already asked her about it, alongside a sketch, and now understands that Kaga is investigating the murder of Michiko Oshitani. Kaga explains that he is pursuing this particular angle because the bridges fall within his precinct. Ichikokubashi Bridge therefore sits within a chain of inquiry: a repeated written pattern prompts questions about Koshikawa, links to Kaga’s family papers, and helps bring the investigation into conversation with Hiromi Kadokura. It remains a clue whose repetition is established, while its intended meaning and connection to any specific visit are left uncertain.