A casing for housing an occupant protection device control unit is mounted to a vehicle body. The casing is composed of a metallic base mounted on the vehicle body with its plate surfaces facing upward and downward, respectively, and a main body that is put on and mounted on the base. The base has a lock portion that is deformable according to a deformation of the vehicle body caused by a collision load from a vehicle front. The lock portion, when deformed, causes a front portion of the base to be locked with at least a part of a front wall of the main body.