Disclosed is a method for producing a modified propylene polymer that exhibits low flowability and also exhibits little fluctuation in melt flow rate compared with the melt flow rate of the propylene polymer before modification, the method involving a heat treatment step of subjecting a mixture comprising 100 parts by weight of a propylene polymer (A), from 0.1 to 50 parts by weight of an ethylenically unsaturated bond-containing compound (B), and from 0.01 to 20 parts by weight of an organic peroxide (C) whose decomposition temperature at which the half-life thereof becomes 1 minute is lower than 120° C. to heat treatment by using an extruder at a temperature lower than the decomposition temperature of the organic peroxide (C) at which the half-life thereof becomes 1 minute.