Disclosed is a method for producing a modified propylene polymer, the method comprising heating a mixture of 100 parts by weight of a propylene polymer (A) defined below and from 0.1 to 50 parts by weight of an ethylenically unsaturated bond-containing monomer (B) in the presence of from 0.01 to 20 parts by weight of an organic peroxide (C); propylene polymer (A) being a propylene polymer composed of from 0.5 to 90% by weight of a propylene polymer component (A1) having an intrinsic viscosity [η], as measured in tetralin at 135° C., of from 5 dl/g to 15 dl/g and from 10 to 99.5% by weight of a propylene polymer component (A2) having an intrinsic viscosity [η], as measured in tetralin at 135° C., of not less than 0.1 dl/g but less than 5 dl/g.