A valve drive system comprises a power transmitting mechanism (13) that converts rotary motion of an electric motor (12) into opening and closing motion of an intake valve (3) provided in a cylinder (2) of an internal combustion engine (1) to transmit power from the electric motor (12) to the valve (3) via a cam (152); and a rotational angle restricting mechanism (16) that is provided in a motion transmission path that extends from the electric motor (12) to the cam (152) and restricts rotation of the cam (152) within a predetermined angular range that is set so that a piston (5) of the engine (1) and the intake valve (3) do not interfere with each other. The rotational angle restricting mechanism (16) comprises a flange (161) that rotates as a unit with a camshaft (151) and forms a slotted groove hole (161a) thereon; and a stopper pin (162) that is inserted into and retracted from the groove hole (161a).