A lock mechanism includes: a lock portion that protrudes from a ceiling wall (wall portion) of a cover and has a locking claw provided on a tip side; a lock receiving portion provided in a case and formed with a locking hole; and a flexible deformation portion which is a portion of a predetermined range in the vicinity of the lock portion on the ceiling wall (wall portion) of the cover and has flexibility higher than a portion outside the predetermined range, and is bent such that the lock portion is tilted so that the locking claw gets over an edge of the locking hole when the locking claw is locked to the locking hole, and after the locking, biases the lock portion in a pulling direction that pulls the locking claw together with the locking hole by a restoring force of bending.