To provide a tool for fitting a snap button for easily and surely caulking a snap button part with a hammer and a punch.

With this tool 1 for fitting a snap button, a part of the snap button is caulked by the punch P to fit the snap button to a cloth piece. The tool has a base member 2 and a guide member 4. The base member includes a snap button receiving part for disposing a snap button to be fitted to the cloth piece C, and a guide member fitting part for removably assembling the guide member in a designated position. The guide member includes an engagement part engaged with the guide member fitting part to assemble the guide member to the base member in a designated position, an arm member 34 for assembling the cloth piece in a space up to the base member, and a punch guide part 32 vertically guiding the caulking punch in a designated position of the snap button disposed in the snap button receiving part.