Lock mechanisms are provide between a first case part and a second case part, which are openable and closable. Projections are formed on an outer side surface of the first case part. Each projection has a stopper hole that extends along a circumferential direction of the outer side surfaces of the case parts. U-shaped deformable pieces are formed with the outer side surface of the second case part and each deformable piece can be deformed in a direction perpendicular to the outer side surface of the second case part. A hook portion is formed with a distal portion of each deformable piece. Guide portions are formed at an edge of an entrance of the stopper hole and guide the hook portion toward the stopper hole. Holding portions are formed at an edge of an exit of the stopper hole and hold claw portions of the hook portion.