The culprit is police officer, Sumio Tateno. The calendar found in the victim's home had unusual markings nest to certain dates of the month. The markings showed the victim's visits and when his blackmail money was expected. The codes are a mixture of numbers and letters that he uses to represent words. Conan figured this out after seeing Ooka's notebook that read "Maru -oka" . He realized that it was not a circle, but a "0" in front of "oka" (in Japanese) making it "Ooka". Katsutoshi Kusaka is "9A". Kusaka can be read as "kyuu A" or "9A" in Japanese). Kiwako Kaitani, who was a pharmacist was 89 "woman" (in Japanese) because ("ya-kyuu") in Japanese sounds like "pharmacist" ("yakuzaishi") and becomes 89. She is a female so he adds "woman". Furthermore, "403" or "yonhyaku san" sounds like "yon go san", referring to Shiomi Hotel. Also, Ooka wrote "100" when he went to the pharmacist because it sounds like "pharmacy". "K" was for "keikan", policeman in Japanese.
Conan thought it was funny when the officer said he would look to see if there was anyone around and he went straight to the kitchen rather than checking the room across the hallway. At the time, the office had gone to lock the back entrance which he came in through to murder Ooka. It is revealed that he was the one rooting throught he neighborhood garbage while he was getting information on Ooka. This way he was allowed to patrol the area frequently without being suspicious. While Tateno had murdered Ooka, his hat dropped onto the spilled sugar. Conan noticed Tateno cleaning off sugar from his hat at the police station and saw that he dusted the hat off upside down. Conan knew the substance was sugar because ants had begun to gather around that area. Conan concluded that Ooka's missing false tooth must have also fallen into Tateno's hat during the struggle. The tooth was found in his police hat.
Tateno revealed that he was also one of the lucky members that escaped arrest for playing mahjong. He decided to focus on school afterwards. Ooka recognized him and started to blackmail him. Ooka also increased the rate when he realized Tateno was going to be promoted into the crime department. Once Tateno realized Ooka was blackmailing others as well, he decided to kill in the name of "justice".
In the end, Kusaka was convicted of assault and Kaitani was convicted of theft.
