To improve workability in assembly of a shutter while preventing the occurrence of looseness.

With a spring member 80 housed in a spring housing part 67, a shutter 50 is assembled on a cover 60 and then, the shutter 50 is rotated from this assembling position toward an open position. By this a spring engagement part 58 presses the spring member 80 and a biasing force is accumulated in the spring member 80. Furthermore, when the shutter 50 is rotated in a range not reaching the open position, a rotation regulation part 56 is locked with a rotation regulation receiving part 66, and the shutter 50 is held by the cover 60 with the energizing force by the spring member 80 working. Thereby, looseness of the shutter 50 is suppressed.