Abstract: A conjugate cam reducer includes input and output units disposed at two opposite sides of a transmission unit. The transmission unit includes smaller-diameter and larger-diameter cam discs axially connected with each other. The smaller-diameter and larger-diameter cam discs have first and second grooves. The input unit includes an input disc, an eccentric shaft and a plurality of input rollers. The input disc has a smaller inner peripheral wall engaging with the smaller-diameter cam disc, and a plurality of first receiving grooves registered with the first grooves to receive the input rollers. The eccentric shaft is rotated to drive rotation of the transmission unit in an eccentric cycloidal motion. The output unit includes an output disc having a larger inner peripheral wall which engages with the larger-diameter cam disc, and a plurality of second receiving grooves which are registered with the second grooves to receive a plurality of output rollers.