A focusing plate adjusting mechanism includes a focusing plate which is detachably attached to a camera body and on which an object image is formed, an adjusting frame which is adapted to adjust the axial position of the focusing plate, a biasing spring which presses the focusing plate against the adjusting frame, and at least one holder which is moved in association with the adjusting frame when the focusing plate is not attached and which presses the focusing plate toward the adjusting frame through the biasing spring when the focusing plate is attached to the camera body.