An automatic speed reducing ratio-switching apparatus comprising an input side carrier, an output side carrier, and an intermediate carrier includes first and second planetary gear mechanisms juxtaposed in the axial direction of an input shaft, first and second inner clutch members which are rotatable in one direction while locking rotation in the other direction of the input shaft, and have mutually different locking directions, first and second outer clutch members which are rotatable in one direction while locking rotation in the other direction of the input side or output side ring gear, and have mutually different locking directions, and a viscous resistance member functioning under a static frictional force to integrally rotate the input side or output side sun gear, the input side or output side planet gears, and the input side or output side ring gears respectively in an identical direction.