Yoroibashi Bridge is one of twelve bridges in the Nihonbashi area whose names appear, month by month, on Mutsuo Koshikawa’s calendar and on a separate list found in Kyoichiro Kaga’s mother’s apartment. The matching entries make the bridge part of a physical clue that connects an unresolved family history with the detectives’ investigation.

Place in the Twelve-Bridge List

The list assigns Yoroibashi Bridge to September. On Koshikawa’s calendar, it appears alongside eleven other Nihonbashi-area bridges, one on each monthly page. The same sequence appears on a sheet of paper Kaga found among the effects in his mother’s apartment. The exact correspondence between the calendar and the note makes the bridge name significant not as an isolated annotation but as one component of a repeated twelve-part pattern.

The search team considered whether Koshikawa had attended events held on the bridges, but its inquiries found no such events. The notes therefore remained unexplained during the initial investigation. Their repetition across the calendar and Kaga’s note gave the detectives a concrete connection to pursue without yet clarifying what the bridge sequence meant.

Connection to Kaga’s Family Mystery

Kaga cautioned that the note’s presence in his mother’s apartment did not prove that it had belonged to her: its handwriting was plainly not hers. He told his superiors that his mother had been in a long-term relationship with Shunichi Watabe and suspected that Watabe might have written the list. Since Kaga knew little about Watabe and had been unable to interpret the note, he asked the task force to arrange handwriting analysis. The matching bridge names thus brought an obscure object from his mother’s private life into the police inquiry.

The investigation later established that Hiromi Asai and Watabe had been connected for at least ten years, while Watabe appeared to have had few other close connections. Kaga reasoned that Hiromi may have supported him financially and that such a sustained relationship suggested either a lover or a close relative. Taking Watabe’s estimated age, his disappearance, and his acquaintance with Michiko Oshitani into account, Kaga concluded that Watabe was probably Hiromi’s father, Tadao Asai. The bridge list does not by itself prove that identity, but its presence among Kaga’s family papers is part of the clue chain that leads the investigation toward it.

Bridge Route and Investigative Use

The detectives also encountered Yoroibashi Bridge as a real point on the waterway route corresponding to the names under examination. During a boat trip, they passed beneath Edobashi Bridge, Yoroibashi Bridge, Kayababashi Bridge, Minatobashi Bridge, and Toyomibashi Bridge before reaching the Sumida River and turning back into the Kanda River. This route gave the list a geographical dimension: its entries could be followed across a connected series of bridges rather than treated only as words on paper.

Yoroibashi Bridge’s narrative importance lies in this double presence. It is both a named entry in a recurring pattern and a location along the detectives’ route through the Nihonbashi waterways. The bridge helps make the clue spatially legible, while the unexplained list keeps Kaga’s personal search and the wider case joined without prematurely resolving what the annotations were intended to signify.