Cutting tool and processing method by the same

A processing method for processing a processed member having an opening portion with a curved inner surface by a cutting tool which includes a shank member and a processing member having a smaller diameter than the shank member, the processing method comprising:

• a first process for forming a through hole at the processed member by a groove portion of the processing member, the through hole extending from a surface of the processed member to reach the opening portion,
• the groove portion having a cutting edge which is positioned at a tip of the groove portion and a chip evacuation groove which is positioned at an outer surface of the groove portion and through which chip generated in the processing is expelled outward;
• and
• a second process which is successively performed after the first process to process a part of the inner surface of the opening portion corresponding to the through hole by the groove portion in a state where a self guide portion of the processing member is guided by the through hole,
• the self guide portion being provided between the groove portion and the shank member of the cutting tool and having a substantially cylindrical shape to guide the cutting edge toward the part of the inner surface of the opening portion corresponding to the through hole.