An internal combustion engine comprising: a driving power transmitting shaft rotatably supported to a crankcase and extending outside from the crankcase, the driving power transmitting shaft having a tip end covered with a clutch cover joined to the crankcase; an outer member located between the crankcase and the clutch cover, and supported relatively rotatable to the driving power transmitting shaft; a center member supported relatively unrotatable to the driving power transmission shaft inside the outer member; friction members supported to the respective outer member and center member relatively displaceable in an axis direction, a mutual contact in the friction members providing a friction force to transmit a rotational force from the outer member to the center member; a pressure member supported to the center member between a first position and a second position to be relatively displaceable in the axis direction, the mutual contact in the friction members being restrained at the first position by receiving an elastic force from a spring member, the friction members being released, at the second position, from the mutual contact against the elastic force; a manipulation member located between the crankcase and the clutch cover to be relatively displaceable in the axis direction, the manipulation member forcibly displacing the pressure member from the first position to the second position according to the axial displacement; and a lifter plate member, characterized in that the lifter plate member is coupled to the center member in the axis direction at a fulcrum (Ls) that is away by a first distance from a force point (Lf) at which a displacement of the manipulation member is received, wherein the lifter plate member is further coupled to the pressure member in the axis direction at an acting point (Lw) that is away from the fulcrum (Ls) by a second distance shorter than the first distance. a driving power transmitting shaft rotatably supported to a crankcase and extending outside from the crankcase, the driving power transmitting shaft having a tip end covered with a clutch cover joined to the crankcase; an outer member located between the crankcase and the clutch cover, and supported relatively rotatable to the driving power transmitting shaft; a center member supported relatively unrotatable to the driving power transmission shaft inside the outer member; friction members supported to the respective outer member and center member relatively displaceable in an axis direction, a mutual contact in the friction members providing a friction force to transmit a rotational force from the outer member to the center member; a pressure member supported to the center member between a first position and a second position to be relatively displaceable in the axis direction, the mutual contact in the friction members being restrained at the first position by receiving an elastic force from a spring member, the friction members being released, at the second position, from the mutual contact against the elastic force; a manipulation member located between the crankcase and the clutch cover to be relatively displaceable in the axis direction, the manipulation member forcibly displacing the pressure member from the first position to the second position according to the axial displacement; and a lifter plate member,