To prevent a flaw of a cutting hole inner wall by chips generated at reamer work time.

A cutting edge 14 and a guide pad 15 are arranged in an edge part, and a coolant supply hole 18 formed in a shank part is opened by the edge part 14 and an outside diameter part of the guide pad 15. A diameter D2 of the guide pad 15 is formed in a smaller diameter than a diameter D1 of the edge part 14, and a clearance is arranged between a cutting hole inner wall of a work whose diameter is expanded by the edge part 14 and the outside diameter part of the guide pad 15. A prescribed quantity of coolant is supplied to the clearance, and a coolant layer is formed. Rotary motion of a reamer is self-guided by coolant pressure of the coolant layer. Therefore, even if the guide pad 15 is not slidingly contacted with the cutting hole inner wall, rotary motion of the reamer can be self-guided, and hole accuracy is excellently held, and since chips are discharged together with a coolant without being rubbed against the cutting hole inner wall, there is no possibility of damaging the cutting hole inner wall.