A cutting tool for machining a differential case, the cutting tool being suitable for machining a differential case with shaft holes that are a pair of through-holes and a pair of openings in a right-left direction of the side surface, axle holes that are a pair of through-holes in an up-down direction, and a spherical inner surface through which both of the pair of shaft holes pass, the cutting tool being suitable for being attached to a machining apparatus in which a tool base to which the cutting tool is fixable can be moved in the up-down direction and the right-left direction and which is equipped with a rotary body for rotating the differential case, and the cutting tool being suitable for performing cutting on the spherical inner surface of the differential case, the cutting tool comprising: a shaft part; a fixing part formed at one end side of the shaft part and that is configured for being fixed to the tool base of the machining apparatus; and a retaining part formed at the other end side of the shaft part and that is configured for being by the machining apparatus, wherein the shaft part is configured to enter the differential case from the opening together with the cutting edge fixed to the shaft part, and the cutting tool is configured to perform cutting on the spherical inner surface through which both of the pair of the shaft holes of the differential case pass, in a both-ends-held state in which both the fixing part and the retaining part are supported by the machining apparatus, without rotating the shaft part and the cutting edge, while the differential case is rotated integrally with the rotary body and while the retaining part is fixed to the tool base and moved integrally therewith in the up-down direction and the right-left direction, characterised in that the cutting tool further comprises an arcuate cutting edge immovably fixed to the shaft part and laterally protruding from the shaft part. the cutting tool being suitable for machining a differential case with shaft holes that are a pair of through-holes and a pair of openings in a right-left direction of the side surface, axle holes that are a pair of through-holes in an up-down direction, and a spherical inner surface through which both of the pair of shaft holes pass, the cutting tool being suitable for being attached to a machining apparatus in which a tool base to which the cutting tool is fixable can be moved in the up-down direction and the right-left direction and which is equipped with a rotary body for rotating the differential case, and the cutting tool being suitable for performing cutting on the spherical inner surface of the differential case, the cutting tool comprising: a shaft part; a fixing part formed at one end side of the shaft part and that is configured for being fixed to the tool base of the machining apparatus; and a retaining part formed at the other end side of the shaft part and that is configured for being by the machining apparatus, wherein the shaft part is configured to enter the differential case from the opening together with the cutting edge fixed to the shaft part, and the cutting tool is configured to perform cutting on the spherical inner surface through which both of the pair of the shaft holes of the differential case pass, in a both-ends-held state in which both the fixing part and the retaining part are supported by the machining apparatus, without rotating the shaft part and the cutting edge, while the differential case is rotated integrally with the rotary body and while the retaining part is fixed to the tool base and moved integrally therewith in the up-down direction and the right-left direction, characterised in that the cutting tool further comprises an arcuate cutting edge immovably fixed to the shaft part and laterally protruding from the shaft part. a shaft part; a fixing part formed at one end side of the shaft part and that is configured for being fixed to the tool base of the machining apparatus; and a retaining part formed at the other end side of the shaft part and that is configured for being by the machining apparatus, wherein the shaft part is configured to enter the differential case from the opening together with the cutting edge fixed to the shaft part, and the cutting tool is configured to perform cutting on the spherical inner surface through which both of the pair of the shaft holes of the differential case pass, in a both-ends-held state in which both the fixing part and the retaining part are supported by the machining apparatus, without rotating the shaft part and the cutting edge, while the differential case is rotated integrally with the rotary body and while the retaining part is fixed to the tool base and moved integrally therewith in the up-down direction and the right-left direction, characterised in that the cutting tool further comprises an arcuate cutting edge immovably fixed to the shaft part and laterally protruding from the shaft part.