Cutting tool and cutting method

A cutting tool, comprising:

• a shaft portion;
• and
• a blade portion provided on a side surface of the shaft portion, wherein
• the blade portion includes cutting blades arranged in a plurality of lines on the side surface of the shaft portion along a peripheral direction, the cutting blades being arranged in a plurality of stages in an extending direction of a shaft center of the shaft portion in each line,
• the cutting blades each have a radial-direction clearance angle inclined inward in a radial direction from a blade edge with respect to a tangential line of the side surface of the shaft portion, a tip end-side clearance angle made such that a tip end-side surface facing a tip end side in the extending direction of the shaft center of the shaft portion is inclined from the blade edge to a base end side with respect to the peripheral direction, and a base end-side clearance angle made such that a base end-side surface facing the base end side in the extending direction of the shaft center of the shaft portion is inclined from the blade edge to the tip end side with respect to the peripheral direction, and
• the number of lines B of the cutting blades falls within a range of B =360/θ or more, where a rotating speed of the shaft portion is V, a cutting arc length of one blade is L2, an angle θ is a rotation angle of the shaft portion when L2/V <1.0×10−3 is established.