It is an object of the present invention to provide a resin powder composed of an isobutylene polymer having low contents of a remaining solvent and a remaining monomer in a product. The present invention is accomplished by a method of removing a solvent while suspending the isobutylene polymer solution in water with a surfactant to obtain resin slurry, and subjecting the resin slurry to steam stripping at a temperature ranging from 150° C. to less than 180° C. to produce a resin powder without deterioration in quality, the resin powder having significantly low contents of the remaining solvent and remaining styrene in the resin.