To provide a locking mechanism such that a front mask can be easily attached and detached even if right and left work spaces of a housing are narrow.

An inner cylinder 104 of a lock 102 provided at one width-directional end part of a front mask 1 is rotated with a key so as to engage an engagement piece 23 formed at an outer peripheral part of the inner cylinder 104 with a lock part 22 at one width-directional end part of the housing 2, and an operation lever 3 for front mask fixation abutting on a cam surface 26 of a cam piece 24 at an outer peripheral part of the inner cylinder 104 is moved along the width direction of the front mask 1 so as to engage the other end part 3a with a lock part 27 at the other width-directional end part of the housing 2. With a configuration in which the lock 102 having the engagement piece 23 and the operation lever 3 for front mask fixation are arranged on one straight line, a locking mechanism is fitted to the front mask 1 which is small in height and thickness in a direction orthogonal to a longer direction.