An accessory unit mountable on an eyepiece portion includes an engaging member having an engaging claw portion engageable with the eyepiece portion, an operating portion configured to manipulated by a user so that the engaging claw portion is displaced in a direction of releasing the engagement of the engaging claw portion and the eyepiece portion, and a mechanism portion configured to displace the engaging claw portion in a direction such that the engaging claw portion and the eyepiece portion are engaged with each other when a force is applied to the accessory unit mounted on the eyepiece portion in a direction opposite to the mounting without manipulating the operating portion.