An internal combustion engine, which comprises: a camshaft having a gear section and a cam section both formed integrally with the camshaft; and a decompression mechanism for securing activation of the internal combustion engine, the decompression mechanism including: a pair of supporting protrusions (41,42) provided on the gear section and each having an insertion hole formed therethrough; a support shaft inserted through the insertion holes of the pair of supporting protrusions (41,42); a decompression member pivotably supported on the support shaft and having a pair of projections (71,72) disposed between the pair of supporting protrusions (41,42), the decompression member being movable toward and away from the cam section; and a biasing spring supported on a portion of the support shaft for normally biasing the decompression member toward the cam section, the camshaft also having: a guide recess formed in a side of the camshaft opposed to the decompression member and between the gear section and the cam section so that the decompression member is slidable along the guide recess; characterised in that the camshaft further comprises: a metal core rod insert formed centrally in the camshaft; and a balancing recess formed in another side of the camshaft opposite the guide recess, the metal core rod being exposed through the guide recess and the balancing recess; and in that the portion of the support shaft on which the biasing spring is supported is a portion between the pair of projections (71,72). a camshaft having a gear section and a cam section both formed integrally with the camshaft; and a decompression mechanism for securing activation of the internal combustion engine, the decompression mechanism including: a pair of supporting protrusions (41,42) provided on the gear section and each having an insertion hole formed therethrough; a support shaft inserted through the insertion holes of the pair of supporting protrusions (41,42); a decompression member pivotably supported on the support shaft and having a pair of projections (71,72) disposed between the pair of supporting protrusions (41,42), the decompression member being movable toward and away from the cam section; and a biasing spring supported on a portion of the support shaft for normally biasing the decompression member toward the cam section, a pair of supporting protrusions (41,42) provided on the gear section and each having an insertion hole formed therethrough; a support shaft inserted through the insertion holes of the pair of supporting protrusions (41,42); a decompression member pivotably supported on the support shaft and having a pair of projections (71,72) disposed between the pair of supporting protrusions (41,42), the decompression member being movable toward and away from the cam section; and a biasing spring supported on a portion of the support shaft for normally biasing the decompression member toward the cam section, a guide recess formed in a side of the camshaft opposed to the decompression member and between the gear section and the cam section so that the decompression member is slidable along the guide recess; a metal core rod insert formed centrally in the camshaft; and a balancing recess formed in another side of the camshaft opposite the guide recess,