A stop mechanism, including: diaphragms; a reception receiving the diaphragms; a cam rotating the diaphragms to change an aperture diameter; and an interlocking arm interlocking with rotation of the cam and arranged on a radially outer side of the cam to rotate the cam from an outside of the stop mechanism. The diaphragms each include: an arc portion defining the aperture diameter and being received in the reception at maximum aperture; and a light-blocking projection formed on the arc portion to block light outside the aperture diameter. The reception includes cutouts for allowing the light-blocking projection to project outside at the maximum aperture. The interlocking arm is located at a position where the light-blocking projection does not overlap in a circumferential direction, and the light-blocking projection moves into the reception while the interlocking arm moves in the circumferential direction in which the aperture diameter is reduced.