To eliminate the backlash of gears of a tool drive mechanism to enhance the roundness of a pin journal after machined in a device for machining the pin journal while moving a tool corresponding to the eccentric rotation of the pin journal by rotating a crankshaft around a main journal.

The power of a motor 46 is transmitted to rotating spindles 33a, 33b via a pinion gear 48, an idler gear 50 and final gears 52a, 52b in this order. These gears 48, 50, 52a, 52b are all helical gears. The idler gear 50 is divided into a plurality of sub-gears 50b, 50a, 50c in the face width direction, and spacers 50d, 50e are inserted between the sub-gears 50b, 50a, 50c. The thickness of the spacers 50d, 50e is adjusted according to the wear of the gears to substantially adjust the pitch of the idler gear 50 to thereby minimize the backlash between the gears.