Provided is a shutter comprising a main body and a shaft bearing. The main body has a substantially circular plate shape, and, in a closed state to close a passage, a front face of the main body is located upstream of a back face thereof in the passage. The shaft bearing is rotatably engaged with a shaft configured to divide the passage into a main passage and a sub-passage in an open state to open the passage, the sub-passage having a cross-sectional area smaller than that of the main passage. The main body includes: a main-passage-side end located in the main passage in the closed state; and a bent portion bent at the main-passage-side end in a direction from the back face side to the front face side. A parallel cross-section of the main body, the cross-section being taken along a parting plane parallel to the shaft, has a circular arc shape bowed toward the back face side from the front face side.