The object is to efficiently subject steel retainers of needle roller bearings to heat treatment in a short period of time even in small-lot production using compact treatment facilities, thereby producing retainers having improved resistance to wear and surface damage and high in fatigue strength and fracture strength as a whole. The steel retainer 3, which retains needle rollers 2, is formed from a steel plate containing 0.15 to 1.10% by mass of carbon. After forming, the steel plate is subjected to induction heat treatment comprising induction hardening and induction tempering. Such heat treatment can be carried out efficiently in a short period of time, using compact treatment facilities, even in small-lot production. The retainer 3 thus formed is sufficiently resistant to wear and surface damage, and is high in fatigue strength and fracture strength as a whole.