A camera includes a main unit which has an arc-shaped groove and a fixed hook formed on and projecting from the groove; and a bayonet ring which has, on the back face thereof, an arc-shaped projection to be engaged in the groove and a stop hook to go under the fixed hook and stopped by the fixed hook when the projection is engaged in the groove and rotated. The bayonet ring has a mark formed in a position on the front face thereof and a part of the bayonet ring which is furthest away from the mark is lifted by pressing the mark in a state that the bayonet ring is rotated up to a detachment position.