The present invention relates to a clamp mechanism which prevents a tip body from being greatly cut out by a fitting hole to secure strength of a tip without complicating the shape of a head portion of a clamp member or a fitting hole of a tip body. There is provided a clamp mechanism of a tip for pressing the tip, in which a fitting hole is formed through a tip body, by the use of a clamp member having a shaft portion inserted into the fitting hole and a head portion with an outer diameter equal to or larger than that of the shaft portion and thus clamping the tip to a tip fitting seat. In the clamp member, a section, which is perpendicular to the central axis line, of the back surface of the head portion has a circle shape centered at the central axis line. In the tip body of the tip, the head portion of the clamp member can pass through the fitting hole, and a contact portion with which a part of the back surface of the head portion comes in contact at the time of advancing the clamp member is formed in an opening of the fitting hole.