A shutter has two engagement portions projecting from a shielding portion, and extending in an attachment direction of the shutter. A lock portion is formed elastically deformably, and a lock claw portion is provided on a tip portion of each lock portion. The lock claw portion is projected toward an outer side in a width direction intersecting an attachment direction of the shutter. Two different inclined surfaces, a second inclined surface and a first inclined surface, are formed on a tip side of the lock claw portion. According to this arrangement, it becomes possible to realize both weakened sliding resistance between the second inclined surface and the attachment contact portion by minimizing an inclination angle of the second inclined surface, and increased sliding resistance between the first inclined surface and a return contact portion by maximizing the inclination angle of the first inclined surface. Thereby, both shutter attachment and return of the shutter to the initial position can be realized.