Kosuge apartment

The Kosuge apartment is the residence where the investigation found Mutsuo Koshikawa and a puppy calendar marked with bridge names. The apartment’s importance reaches beyond identifying its occupant: the calendar connects Koshikawa’s assumed identity, Shunichi Watabe, to a note among Kyoichiro Kaga’s mother’s effects, while later deductions identify Watabe as Hiromi Asai’s father, Tadao Asai. The apartment thus becomes a point where an apparently isolated occupant’s concealed past enters the murder investigation.

Residence and occupant

The Kosuge apartment was occupied by Mutsuo Koshikawa, the man whose burned corpse was found on the riverbank at Shinkoiwa. Investigators established that Koshikawa had previously lived under the name Shunichi Watabe, a man with a long-term relationship with Kaga’s mother. The apartment therefore marked the place where the investigation encountered one identity, while the occupant’s past tied him to a different name and to Kaga’s family history.

The calendar and the bridge list

Among the apartment’s clues was a flip calendar with one month on each page and pictures of puppies. A bridge name had been written on each monthly page, one per month. The list of bridges began with Yanagibashi Bridge in January and continued through Asakusabashi Bridge, Saemonbashi Bridge, Tokiwabashi Bridge, Ichikokubashi Bridge, Nishigashibashi Bridge, Nihonbashi Bridge, Edobashi Bridge, Yoroibashi Bridge, Kayababashi Bridge, Minatobashi Bridge, and Toyomibashi Bridge in December.

Kaga’s mother had a separate note in her apartment listing the same bridge names against the same months. Kaga cautioned that the note’s ownership was uncertain: it had been found among her effects, but its handwriting was clearly not hers. The exact match with the calendar at the Kosuge apartment made the two objects a significant investigative parallel, without by itself establishing who wrote either one or why.

From a location to a concealed identity

The calendar helped focus attention on the connection between Koshikawa and Watabe. Kaga explained to his superiors that his mother had maintained a long-term relationship with a man named Shunichi Watabe, and he suspected Watabe had written the matching bridge list. At that stage, however, he knew almost nothing about Watabe or what the note meant. The apartment’s clue was therefore part of a puzzle whose personal significance remained unresolved.

The investigation had initially concentrated on finding someone known to both Hiromi Asai and Michiko Oshitani who had disappeared, and had given substantial attention to Seizo Naemura. After testimony established that Naemura was not the person they sought, Kaga reconsidered the evidence. He reasoned that Hiromi Asai had been connected to Watabe for at least ten years and might have supported him financially; given Watabe’s estimated age, his disappearance, and his connection to Michiko Oshitani, Kaga concluded that Watabe was likely Hiromi’s father, Tadao Asai.

Investigative significance

The apartment’s puppy calendar is a small, concrete clue that links otherwise separate parts of the case: the identity of the man living there, Kaga’s mother’s unexplained note, and the history of Hiromi Asai’s family. Its matching month-by-month bridge list does not immediately solve the mystery, but it gives investigators a material correspondence to follow as they revise their assumptions about Watabe and Naemura. The Kosuge apartment consequently functions as both a crime-scene location and a hinge in the investigation’s reconstruction of Tadao Asai’s hidden identity.