To provide a cutter knife capable of reversing a tail tap in a proper direction according to a dominant hand for stable holding.

This cutter knife 10 is formed by a handle body 11 extended in the longitudinal direction, a knife edge 12 stored in a passage 20 of the handle body 11, an operating part 13 connected to the rear part of the knife edge 12 to be movable along the passage 20, and the tail tap 13 removably provided on the rear part of the handle body 11. The tail tap 13 has an outline bent in a gentle curve direction to the handle body 11, and is adapted to change the direction of ending according to the direction of the knife edge 12. The bent inner edge of the tail tap 15 is formed as an attitude stable curve part 33, whereby being held, the cutter knife 10 substantially comes into contact with the range from the vicinity of the roots of the thumb and index finger to the back of the hand.