To shorten a machining time of forming a hole on its inner periphery in a work.

A reamer T has chip groups 6... arranging five chips 6 on the smaller circumference than the inner diameter of a hole H in a work W, in three steps with a fixed pitch P to the axial direction. When the reamer T is inserted into the hole H in the work W and then the axial line Lt of the rotating reamer T is moved along the circumference with the axial line Lw of the hole H as a center, the three-step chip groups 6... cut the hole H on its inner periphery as the same time. In such a state, when the reamer T is moved one-pitch from the hole H in a withdrawn direction, the three-pitch chip groups 6... cut the hole H on its inner periphery in the work W at only one pitch P at the same time, so that boring work for the hole H in the work W at a depth of D=3P can be completed in a short time.