To provide an aperture mechanism and an optical apparatus which are reduced in size. The aperture mechanism comprising: a driving member to be driven in accordance with an aperture operation; an aperture member having an aperture to be driven by the driving member; a first engagement section connected to one of the driving member and the aperture member; a second engagement section connected to another one of the driving member and the aperture member and engaged with the first engagement section; and a biasing member connected to at least one of the driving member and the aperture member to provide a bias in a direction in which the first engagement section and the second engagement section are facing to each other.