To easily perform assembly of a camshaft by an inexpensive parts.

While the time drive force is applied to a camshaft input gear 6 from a drive side gear 7, backward thrust 8a is applied to a camshaft 3, this backward thrust 8a is received by a peripheral thickness part 9 of a front bearing hole 1 through the camshaft input gear 6, a collar 10 is fixedly externally fitted to the camshaft 3 in the backside of a rear bearing hole 2, so as to receive a forward thrust 8b, applied to the camshaft 3, stopped by a peripheral thickness part 11 of the rear bearing hole 2 through the collar 10. Accordingly, the forward thrust 8b is only instantaneously applied to the collar 10, so that the collar 10 can do with simple structure not requiring solidity and lower in part cost.