To provide a barrel worm-shaped tool capable of grinding processing in high precision, while maintaining the reduction of heavy load during the processing and partial wear. For this purpose, a barrel worm-shaped tool that is used in the gear machining of an internal gear (11) is formed with its diameter gradually increasing from the two end portions in the axial direction thereof towards the middle portion thereof. The barrel worm-shaped tool (12) is formed so that edge shapes (31) of the grinding wheel (12) becomes gradually smaller from the edge shape (31) located in the middle portion, in the axial direction of the barrel worm-shaped tool (12) towards the edge shape (31) located in the two end portions, in the axial direction, of the barrel worm-shaped tool (12) in accordance with the amount of machining allowance for the internal gear (11).