2 Nen a Gumi Tantei Kyoku [3] (Kadokawa Tsubasa Bunko)

[Machine Translation] A boy who had been bullied came to junior high school wearing a mask. Then, he changed into a cheerful personality as if he had become a different person! It became news, and the masks went viral in schools all over the country! However, a murder occurs... Ariki and Mitsugi sneak into the masked assembly and investigate the conspiracy hidden behind it. Our Eiji and Aihara and the others cooperate with the investigation! This is the third installment of Osamu Soda's Mystery 2A Detective Bureau. From elementary school upper grade.

いじめられていた少年が仮面マスクをして中学校にやってきた。すると、別の人になったように明るい性格に変わっていた!ニュースとなり、マスクは全国の学校に大流行!?ところが、殺人事件が発生して...。有季と貢は、仮面の集会にもぐりこみ、裏にかくされた陰謀を探る。ぼくらの英治と相原たちも捜査に協力!宗田理のミステリー2A探偵局、第3弾。小学上級から。