To facilitate screwing-in of a clamp screw and a female screw part and prevent a drop of the clamp screw when a small tip is clamped at a vertical type tool holder by a small clamp screw.

A tip shaft part 13 is protruded from the tip of the male screw part 12 of a clamp screw 10. Prior to clamping of a clamp screw 10, the tip shaft part 13 enters a female screw part 4. By fastening the clamp screw 10 after a temporary mounting state described above, a drop of the clamp screw 10 is prevented from occurring and a tip 3 is easily clamped.