A lock mechanism holds a moving member on a main body immovably in a locked state and allows movement in an unlocked state. The lock mechanism includes a circulating cam groove formed on the main body or the moving member, and a trace member having an angle bracket shape with one and the other trace parts to sway on the main body or moving member. The one trace part circulates along the circulating cam groove in each approaching and moving away operation of the moving member, and locks the moving member in a state having coupled with the coupling part. A wall surface is formed on the main body or moving member to be disposed in a different position from the circulating cam groove so that the other trace part of the trace member is operable to contact the wall surface only in the locked state of the moving member.