PROBLEM TO BE SOLVED : To provide a method for producing hydrogel by radiation crosslinking in which hydrogel having a particle diameter in a specific range is optionally selected out of a wide range of particle diameters including a small particle diameter, which could not be achieved in a conventional method. SOLUTION : In a method for producing hydrogel particles by irradiating aqueous solution containing hydrophilic polymer with radiation, dissolved oxygen concentration in the solution is adjusted, and/or the hydrophilic polymer having molecular weight in a specific range is selected to control the particle diameters of the hydrogel particles. SELECTED DRAWING : None COPYRIGHT : (C)2019, JPO&INPIT