To provide a reducer low in lost motion at low cost.

In an eccentric oscillation type reducer 1 activated with a crankshaft 3 as an input shaft and a carrier 4 as an output shaft; two pairs of torque transfer systems are constituted using two inner gears 21, 91 and two external gears 5, 6; lost motion in an engage part of the inner gears 21, 91 and the external gears 5, 6 is eliminated by making each of them exclusively for normal rotation and reverse rotation, respectively, and continuously in contact with a force transmission gear tooth surface. With coupling 7 of structure where a first movable plate 72, in which the carrier 4 and the external gear 5 are movable to a fixed plate 71 in a direction toward an X axis, restrained in the rotation direction, and a second movable plate 73, movable to the first movable plate 72 in the direction toward a Y axis, restrained in the rotation direction, the device is coupled. By fixing the fixed plate 71 to the carrier 4 and fixing the second movable plate 73 to the external gear 5, orbital motion of the external gear 5 is absorbed, and only the rotation movement is transferred to the carrier 4 without the lost motion.