To provide an O-ring groove capable of preventing drop of an O-ring through simple structure, and contributing to improve quality of a tapping tool and assembly workability.

A tapping tool 10 is provided with a body 31. An O-ring groove 30 is formed in the inner end face 34 of the second cylinder part 22 of the body 31. A protrusion part for preventing a drop of the O-ring is formed on the mouth part of the O-ring groove 30. The protrusion part is protruded in an inner peripheral direction from the outer peripheral part of the groove, and a section is formed in a thin triangle shape toward an apex. An O-ring 20 is incorporated in the O-ring groove 30. Since the O-ring 20 is pressed by a protrusion part 37, the O-ring is prevented from dropping from the interior of the O-ring groove 30.