To provide a method for machining a differential case inner surface capable of eliminating fixing work of a machining tool and a mounting tool in a differential case by allowing the machining tool to be put in and out from the inside of the differential case through the one hole thereof and miniaturizing a machining device for machining the inner surface of the differential case, and also to provide a means for machining the inner surface of the differential case.

The means 10 for machining the differential case inner surface is comprised of: the machining tool 18 comprising a main shaft part 12, a protruding tip part 14 laterally protruding from the main shaft part 12, and a counterboring tool 16 fixed to the protruding tip part 14; and a guide bush 22 movably fitted to the main shaft part 12. The counterboring tool 16 is inserted into an inner space 38 from the pinion hole 44 of the differential case 34 and the guide bush 22 is fitted to the pinion hole 44. Thereafter, the counterboring tool 16 is made to contact with the spherical seat 46 of the differential case 34 while the machining tool 18 is rotated, and the spherical seat 46 is machined.