An internal combustion includes: a camshaft (24) having a metal core rod (52) insert formed centrally in the camshaft and a driven gear (23), an exhaust cam (25), etc. formed of resin integrally with the camshaft,and a decompression mechanism (20). The decompression mechanism (20) includes: a pair of supporting protrusions (41,42) provided on the driven gear and each having an insertion hole (57),a decompression member (45) having a pair of projections (71,72) disposed between the supporting protrusions (41,42),and a biasing spring (47) disposed between the projections for normally biasing the decompression member toward the exhaust cam (25). The camshaft (24) also has a guide recess (44) and a balancing recess (45) formed in opposite sides of the camshaft, and the metal core rod (52) is exposed through the guide recess and the balancing recess.