This invention provides a retainer assembling apparatus which assembles a retainer to a valve stem via a cotter which engage with an engaging groove formed on the valve stem. The cotter comprises a pair of cotter segments. The apparatus includes a cylindrical body, a holding portion which holds the retainer contacting a lower end of the cylindrical body, a push-up shaft which pushes up the pair of cotter segments into the cylindrical body, and an axial body which is arranged in the cylindrical body to be coaxial with the push-up shaft and to be coaxially movable. The cylindrical body includes a tapered inner circumferential portion on an inner circumferential wall lower portion thereof.