To easily modify the diameter of a round bar shaped member such as a pin by forming a cavity portion longitudinally the bar shaped member and in the central part of the bar shaped member, providing a cutting blade for cutting to the inner side of an opening of the cavity portion, and inserting an object to be cut into the cavity portion to cut it.

A cavity portion 2 is longitudinally formed in the central part of a bar shaped member 1, cutting blades 4 for cutting are provided for an opening portion 3 of the cavity portion 2, and thereby an outer cutting tool 5 is formed. The plurality of blade 4 are placed so as to face and surround the cavity portion 2. When an object to be cut such as a socket pin is cut and its outer diameter is modified using such outer cutting tool 5, the cutting tool 5 matching with the diameter of the socket pin 7 is grabbed by an electric drill or a chuck of a ball disk, the cutting tool 5 is rotatingly pressed onto the side of the object to be cut, and thereby the outer diameter of the object to be cut is finished with the cutting blade 4 to a given pin size in a short time.