Toyomibashi Bridge is the December entry in a twelve-bridge list written on Mutsuo Koshikawa’s calendar and repeated on a note found among the effects in Kyoichiro Kaga’s mother’s apartment. The bridge also appears on the boat route used by the detectives as they traced the list through the waterways of Nihonbashi, giving this otherwise unexplained annotation a physical place in the investigation.

Location in the Bridge List

The calendar carries one bridge name on each monthly page. Toyomibashi Bridge is listed for December, after Minatobashi Bridge in November; the sequence begins with Yanagibashi Bridge in January and runs through bridges in the Nihonbashi area. The search team found similar annotations throughout the calendar but could not connect them to any documented events Koshikawa had attended. Their repetition made the list conspicuous without yet explaining its purpose.

The Matching Note

Kaga had found a sheet with the same month-by-month sequence among the belongings in his mother’s apartment. He cautioned that the handwriting was not hers, so its ownership was uncertain. When he showed the list to Shuhei Matsumiya, Matsumiya was astonished by how exactly its places and dates matched the calendar. Kaga told his superiors that his mother had maintained a long-term relationship with Shunichi Watabe and suspected Watabe had written the note, though Kaga did not know what the list meant. Toyomibashi Bridge therefore belongs to a duplicated clue whose writer and intended message were initially unresolved.

The Route on the Water

During a boat trip tracing the listed bridges, the detectives passed Edobashi, Yoroibashi, Kayababashi, Minatobashi, and Toyomibashi before reaching the Sumida River. Instead of continuing along the usual tourist route toward Tokyo Skytree, the boat turned into the Kanda River and eventually returned to Asakusabashi Bridge, where the trip had begun. Toyomibashi Bridge marks the last named bridge on that outward leg, just before the route leaves the Nihonbashi sequence for the Sumida River.

Investigative Significance

The bridge name itself does not explain the calendar or establish that Koshikawa attended an event there. Its significance comes from recurrence: it appears in the same December position on two independently situated lists, one on Koshikawa’s calendar and one in Kaga’s mother’s apartment. As the investigation broadens from the matching lists to the connection between Watabe and Hiromi Asai, the bridge remains part of the concrete pattern that prompts the detectives to reconsider how apparently separate people and clues are linked.