The present invention relates to a reducer (10) comprising: a hollow input shaft (100) having a driving surface (110) therein; an output shaft (200) which is accommodated inside the input shaft (100) and having an output shaft body (210) having an output shaft gear (220) formed along the circumferential direction on the outer surface; a plurality of rollers (R) aligned between the output shaft (200) and the driving surface (110) and extending in the axial direction; and ring-shaped caps (300) positioned on both sides of the axial direction of the rollers (R), wherein the rollers (R) are guided to move in the radial direction by means of the caps (300) and, when the input shaft (100) rotates, the rollers (R) are pressurized against the driving surface (110) to pressurize the output shaft gear (200), so that the output shaft (200) rotates.