There is disclosed a clamping mechanism for a cutting insert that clamps the cutting insert by inserting the cutting insert into an insert mounting seat. The insert mounting seat is formed in a tip of a holder having a pair of contacting surfaces facing each other, and the cutting insert has a pair of contact surfaces to come in contact with the pair of contacting surfaces, respectively. The tip of the holder is provided with a pair of jaws that is formed with the pair of contacting surfaces, respectively, and that can be elastically deformed relatively in a direction that the contacting surfaces contact the contact surfaces, and a clamping means that elastically deforms the pair of jaws to bring relatively the pair of contacting surfaces close to each other and to separate them relatively from each other.