To provide a snap ring assembling device for facilitating alignment of a cap with a shaft.

This snap ring assembling device comprises the cap 20, a cap retainer 30, a cap presser, a snap ring pressing-in part 50, and a transfer part. The cap presser includes a center pin 22 fitted onto a center hole provided on the end surface of the cap 20. With the center pin 22 fitted onto the center hole of the cap 20 for the alignment of the cap 20, a snap ring 1 set at a conical outer periphery of the cap 20 is pushed in to fit the snap ring 1 into an outer-periphery groove of a work 2.