To provide a gear grinding machining method capable of reducing the manufacturing cost by adding a machining function by a worm grinding wheel to a function of a composite machine tool having the flexibility, to integrate the gear grinding machining and the other machining, to reduce the plant and equipment investment, and to reduce a machining time and a moving time between process steps.

In a composite machining NC lathe 1 having a main spindle 13, and a blade table 3 movable to the main spindle 13, a ground gear W on which the grinding machining is to be executed, is mounted on the main spindle 13, the worm grinding wheel 10 is mounted on a tool shaft 11 of a turret head T for gear grinding, the tool shaft 11 is placed with a crossed axes angle θto the main spindle 13 to engage the ground gear W with the worm grinding wheel 10, the ground gear W and the worm grinding wheel 10 are synchronously rotated, and the worm grinding wheel 10 is moved in the direction of the main spindle 13 to execute the grinding machining.