A lock mechanism of a sliding member being slidably movable along a body between an engaged position and a pullout position. The sliding member comprises a face, the face contacting along an outer face of the body in a direction of sliding. The lock mechanism comprises an engaging portion located between the body and the sliding member that prohibits the sliding member from sliding by holding the sliding member at the engaged position by a concavo-convex engagement, and an engagement release mechanism that releases the engaging portion, when an operating force is applied to an operating portion located on the sliding member, to allow the sliding member to slide from the engaged position to the pullout position along a sliding direction and in a direction different from the sliding direction simultaneously. The operating force including components parallel to and orthogonal to the contacting face.