PURPOSE: To make a cam shaft shorter by forming a recessed part which is recessed in the axial direction of said cam shaft, in the center part of a cam sprocket installed on said cam shaft having a cam for driving a mechanical pump (accessory), and installing said cam in said recessed part.

CONSTITUTION: A cam shaft 4 is rotatably supported by a cylinder head 2 with supporting parts 2a..., and is rotated by means of a driving shaft via a cam chain 6 which is put around a cam sprocket 5. A recessed part 7 which is recessed in the axial direction of the cam shaft 4 is formed in the center part on the front side of the cam sprocket 5. On the front side of the cam sprocket 5, a boss member 10 having a mechanical pump cam 8 for driving a mechanical pump 11 and a distributor gear 9 for driving a distributor 12, is installed, while the mechanical pump cam 8 being positioned in the recessed part 7. The mechanical pump 11 sucks in and discharges fluid by means of the swing of a driving lever 11a which is engaged with the mechanical pump cam 8.