A cycloidal speed reducer, comprising: a housing; a rotating shaft extending through the housing in a rotatable manner, wherein the rotating shaft has an input end and an output end, the input end has a first shoulder portion, and the output end has a second shoulder portion; an input flange provided at one end of the housing and mounted around the input end of the rotating shaft; a first bearing having a first inner ring and a first outer ring, wherein the first inner ring is mounted around the input end of the rotating shaft and lies against the first shoulder portion of the rotating shaft, and the first outer ring has an outer peripheral surface abutted against the input flange; an output flange provided at an opposite end of the housing and mounted at the output end of the rotating shaft, wherein the output flange has an adjusting screw hole; a second bearing having a second inner ring and a second outer ring, wherein the second inner ring is mounted around the output end of the rotating shaft and lies against the second shoulder portion of the rotating shaft, and the second outer ring has an outer peripheral surface abutted against the output flange; a speed reducing device having two cycloidal discs and two Oldham's couplings, wherein the cycloidal disc is mounted eccentrically around and connected to the rotating shaft, and one of the Oldham's couplings is provided between the cycloidal discs and the input flange and the other one of the Oldham's couplings is provided between the cycloidal discs and the output flange; and a preload adjustment device having a preloading plate and an adjusting screw, wherein the preloading plate is adjacent to the output end of the rotating shaft and abuts against the second outer ring of the second bearing, and the adjusting screw is threadedly engaged in the adjusting screw hole of the output flange and has one end pressing against the preloading plate such that the preloading plate is pushed by the adjusting screw against the second outer ring of the second bearing and thus applies a preload to the second bearing. a housing; a rotating shaft extending through the housing in a rotatable manner, wherein the rotating shaft has an input end and an output end, the input end has a first shoulder portion, and the output end has a second shoulder portion; an input flange provided at one end of the housing and mounted around the input end of the rotating shaft; a first bearing having a first inner ring and a first outer ring, wherein the first inner ring is mounted around the input end of the rotating shaft and lies against the first shoulder portion of the rotating shaft, and the first outer ring has an outer peripheral surface abutted against the input flange; an output flange provided at an opposite end of the housing and mounted at the output end of the rotating shaft, wherein the output flange has an adjusting screw hole; a second bearing having a second inner ring and a second outer ring, wherein the second inner ring is mounted around the output end of the rotating shaft and lies against the second shoulder portion of the rotating shaft, and the second outer ring has an outer peripheral surface abutted against the output flange; a speed reducing device having two cycloidal discs and two Oldham's couplings, wherein the cycloidal disc is mounted eccentrically around and connected to the rotating shaft, and one of the Oldham's couplings is provided between the cycloidal discs and the input flange and the other one of the Oldham's couplings is provided between the cycloidal discs and the output flange; and a preload adjustment device having a preloading plate and an adjusting screw, wherein the preloading plate is adjacent to the output end of the rotating shaft and abuts against the second outer ring of the second bearing, and the adjusting screw is threadedly engaged in the adjusting screw hole of the output flange and has one end pressing against the preloading plate such that the preloading plate is pushed by the adjusting screw against the second outer ring of the second bearing and thus applies a preload to the second bearing.