The present invention relates to a maleimide group-containing polymer particle containing a crosslinked polymer particle containing a group represented by the following formula (1) and to a method of producing the maleimide group-containing crosslinked polymer particle by reacting hydroxymethylmaleimide with a hydroxyl group-containing polymer particle obtained by introducing a polyethylene glycol into a crosslinked polymer particle: ##STR00001## wherein n denotes an integer of 1 or more.