To provide a gear cutting tool that is mountable on a combined finishing machine, can offer high-precision gear cutting and can improve machining efficiency.

The gear cutting tool 1 is mounted on a combined finishing machine so constructed that it comprises a headstock 2 for journaling a main spindle 3, which has a tool mounting hole 14 with a taper surface 14a, and that a tool mounted in the tool mounting hole 14 is automatically changeable. The gear cutting tool 1 comprises a mounting shank part 4 and a casing 7 for journaling the mounting shank part 4 and journaling a tool shaft 6 mounted with a hob 5. The mounting shank part 4 has a taper shank portion 4b, and the taper shank portion 4b has a taper collar 16 with a taper surface 16a, and an urging means 17 for urging the taper collar 16 to the tip side. For the mounting of the gear cutting tool 1 on the combined finishing machine, the taper shank portion 4b is pressed in the tool mounting hole 14, and a tip end face 35a of an abutting portion 35 of the casing 7 is pressed against a tip end face 2a of the headstock 2.