To provide a reducer capable of preventing vibration generated from a reducing mechanism, preventing uneven rotation by additional fluctuation, and increasing a reduction gear ratio without increasing a size.

This reducer is assembled such that it is constituted of an input shaft 2 for fixing and supporting a planetary roller 1 at its shaft end, an output shaft 4 having a cup-shaped shaft end 3, and a cylindrical case 5 having an inner peripheral length slightly longer than the outer peripheral length of the cup-shaped shaft end 3 for housing the input shaft 2 and the output shaft 4. The planetary roller 1 is press-fitted to the inner peripheral part of the cup-shaped shaft end 3, whereby the outer peripheral part of the cup-shaped shaft end 3 is expanded, and only the expanded part is brought into contact with the cylindrical case 5 by strong pressing force.