To provide a two-stage shifting reduction gear to prevent the genera tion of abnormal vibration and abnormal noise during shifting of a rotation speed.

Low and high speed pinion gears 11 and 12 are supported capable of being in a racing state by a reduction gear casing 1 and a rotation transmission shifting device 40 to transmit rotation of an input shaft 10 to the low or high speed pinion gear 11 or 12 and release transmission of rotation is arranged on the input shaft 10 to loosely fit the shaft holes of the lower and high speed pinion gears 11 and 12 in each other. When rotation of the low speed pinion gear 11 is transmitted to a low speed large gear 21 to rotate an output shaft 20, a high speed large gear 22 is prevented from racing by releasing rotation of the high speed pinion gear 12. Reversely, when rotation of the high speed pinion gear 12 is transmitted to the high speed large gear 22 to rotate the output shaft 20, the low speed large gear 22 is prevented from racing through release of rotation of the low speed pinion gear 11.