A gear grinding method for performing a grinding process on a machining-target internal gear by rotating the machining-target internal gear and a barrel-shaped threaded grinding wheel in mesh with each other, the gear grinding method comprises: forming the threaded grinding wheel by use of first threaded grinding wheels provided on two opposite sides and a second threaded grinding wheel provided between the first threaded grinding wheels; characterized in that the method further comprises: grinding a machining allowance (W2a) of a machining-target internal gear with the first threaded grinding wheels; when a thickness of the machining allowance (W2a) reaches a predetermined thickness as a result of the grinding by the first threaded grinding wheels, causing contact of the first threaded grinding wheels with the machining allowance (W2a) to be smaller than the contact of the first threaded grinding wheels with the machining allowance (W2a) at beginning of the grinding and causing contact of the second threaded grinding wheels with the machining allowance (W2a) to be larger than the contact of the first threaded grinding wheels with the machining allowance (W2a); and grinding the machining allowance (W2a) ground to the predetermined thickness by the first threaded grinding wheels with the second threaded grinding wheel. forming the threaded grinding wheel by use of first threaded grinding wheels provided on two opposite sides and a second threaded grinding wheel provided between the first threaded grinding wheels; characterized in that the method further comprises: grinding a machining allowance (W2a) of a machining-target internal gear with the first threaded grinding wheels; when a thickness of the machining allowance (W2a) reaches a predetermined thickness as a result of the grinding by the first threaded grinding wheels, causing contact of the first threaded grinding wheels with the machining allowance (W2a) to be smaller than the contact of the first threaded grinding wheels with the machining allowance (W2a) at beginning of the grinding and causing contact of the second threaded grinding wheels with the machining allowance (W2a) to be larger than the contact of the first threaded grinding wheels with the machining allowance (W2a); and grinding the machining allowance (W2a) ground to the predetermined thickness by the first threaded grinding wheels with the second threaded grinding wheel. grinding a machining allowance (W2a) of a machining-target internal gear with the first threaded grinding wheels; when a thickness of the machining allowance (W2a) reaches a predetermined thickness as a result of the grinding by the first threaded grinding wheels, causing contact of the first threaded grinding wheels with the machining allowance (W2a) to be smaller than the contact of the first threaded grinding wheels with the machining allowance (W2a) at beginning of the grinding and causing contact of the second threaded grinding wheels with the machining allowance (W2a) to be larger than the contact of the first threaded grinding wheels with the machining allowance (W2a); and grinding the machining allowance (W2a) ground to the predetermined thickness by the first threaded grinding wheels with the second threaded grinding wheel.