An eyecup of an eyecup mounting mechanism includes an eye rubber, a fixing member that holds the eye rubber in a predetermined position, a movable member held movably by the fixing member, and an elastic member that presses the movable member against the fixing member to fix the movable member to the fixing member, and wherein a mounting member provided in the camera body is fitted between the fixing member and the movable member against a pressing force of the elastic member, and the mounting member is sandwiched by the fixing member and the movable member, for the purpose of providing a highly reliable eyecup mounting mechanism and an optical apparatus including the eyecup mounting mechanism.