To increase the rigidity of a timing gear case, especially its central part for less vibration and noise by fixing one end of an idler shaft to which an idler gear is journaled so as to rotate freely on an engine body, and part of a timing gear case to the other end by bolting.
One end of an idler shaft 2 in which an idler gear 6 is inserted so as to rotate freely is pressed in and fixed at a crank case 9, and a mounting surface 1c formed inside a timing gear case 1 where a flange part 16 at the outer periphery is fixed on the crank case 9 by a bolt 11, is brought into contact with the end surface 2a of its other end. By tightening the timing gear case 1 and the idler shaft 2 with a bolt 4, the idler shaft 2 is fixed. It is thus possible to improve the rigidity of the timing gear case 1, especially almost its middle part where the idler gear 6 is mounted, and restrain the timing gear case 1 from vibrating due to the vibration of the end part of the crank case 9 for less noise.