The present invention relates to a speed reducer for a vehicle. The speed reducer includes: a first coupler having, on one side thereof, a first coupling portion, to which a motor shaft or a worm shaft is coupled, and, on the other side thereof, a plurality of first protrusions, which are spaced apart from each other in a circumferential direction while protruding in an axial direction, in which the first protrusions are formed in a manner in which a circumferential width of each of the first protrusions is narrowed toward an inner side from an outer side so as to form a first outer support portion as a radial outer end and a first inner support portion as a radial inner end; and a second coupler configured to be coupled to the first coupler to transmit a rotational force to the first coupler.