Abstract: The present invention relates to a method of removing a residual ethylene oxide monomer in polyethylene oxide used as a raw material for pharmaceutical preparations. More particularly, the method of the present invention comprises retaining high-molecular weight polyethylene oxide particles at a temperature not higher than a crystal melting point of the polyethylene oxide in the absence of an aggregation inhibitor under an atmospheric pressure in the standing state and, then, cooling the particles to 40° C. or lower under an atmospheric pressure and in the standing state, thereby, reducing a residual ethylene oxide monomer concentration to 1 ppm or lower without influencing on physical properties such as a specific surface area of the high-molecular weight polyethylene oxide particles.