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; and a push rod located inside the driving power transmitting shaft to be relatively displaceable in the axis direction, the push rod forcibly displacing the pressure member from the first position to the second position according to the axial displacement, wherein the internal combustion engine further includes: a pin member coupled to the push rod and located in a radial direction of the driving power transmitting shaft between the crankcase and the clutch cover, the pin member radially projecting from an outer periphery of the driving power transmitting shaft; and a lifter member mounted to the outer periphery of the driving power transmitting shaft to be relatively displaceable in the axis direction between the crankcase and the clutch cover, and coupled to the pin member to transmit an axial displacement of the pin member to the pressure member, characterized in that the driving power transmitting shaft has a tip end supported relatively rotatable to the clutch cover, wherein a bearing that rotatably supports the driving power transmitting shaft is incorporated into the clutch cover. 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; and a push rod located inside the driving power transmitting shaft to be relatively displaceable in the axis direction, the push rod forcibly displacing the pressure member from the first position to the second position according to the axial displacement,