A cutting tool is provided to process a processed member having an opening portion with a curved inner surface, so that a through hole is formed and a part of the inner surface of the opening portion corresponding to the through hole is processed. The through hole extends from a surface of the processed member to reach the opening portion. The cutting tool includes a shank member, and a processing member having a smaller diameter than the shank member. A groove portion of the processing member has a cutting edge, and a chip evacuation groove through which chip is expelled outward. A self guide portion of the processing member is arranged between the groove portion and the shank member and provided with 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 in the processing.