An extendable pivot shaft mechanism of a camera includes two concentric cylindrical members fixed to a camera body and a rotatable grip, respectively; an advancing/retreating lock control member supported by one of the concentric cylindrical members and linearly movable along a rotational axis without rotating relative thereto, the advancing/retreating lock control member being operated from outside the camera body or the grip; a rotation locking mechanism for changing a state of the advancing/retreating lock control member and the other of the two concentric cylindrical members between a rotation locked state and a rotation unlocked state in accordance with a linear movement of the advancing/retreating lock control member relative to the one of the concentric cylindrical members; and a lock biasing device for biasing the advancing/retreating lock control member in a rotation locking direction to cause the rotation locking mechanism to change to the rotation locked state.