To provide a working method of a worm gear capable of obtaining an involute shape that roughness of a tooth flank is enhanced and enhancing engagement accuracy of a worm gear.

In the working method of the worm gear 2, a tool of wheel shape having a grinding tool 1 having a cross section of trapezoid shape on an outer peripheral part is used. In a blade part 1a of the trapezoidal grinding tool 1, the respective tooth flanks 2a, 2b of the worm gear 2 is grinding-worked to the involute shape by using the grinding tool 1 in which correction working that it is made to an inner recession shape by slightly making its blade tip and a blade base thick is applied to the existing grinding tool of trapezoid shape.