A method for making a commutator comprising steps of arranging at least one pair of commutator members to be spaced apart from each other, and molding a product by injecting an insulating resin; shaping the product to the product having a real circular sectional cross section; heating the product to a predetermined temperature; cutting protruding resin portions between the commutator members, to the align outer circumference of the commutator body with the outer surfaces of the commutator members. The predetermined temperature is that which rises when the commutator is rotated, and accordingly the predetermined temperature ranges from sixty to eighty degrees Centigrade (60° C.-80° C.). The finished commutator has grooves on the commutator body between the commutator members. The grooves undergo thermal expansion when the commutator is rotated and the temperature rises to a predetermined degree, so that the commutator has a real circular sectional cross section. Accordingly, the contact between the commutator and the input and output brushes is properly performed, and no noise occurs.