Machining method and machining apparatus of valve holes of cylinder head, and clamp device

A clamp device comprising:

• a body section rotatable around a rotary shaft;
• a rotary unit which adjusts a position at which the body section rotates;
• a stopper section fixed to one end of the body section in an axial direction;
• an urging device which urges the body section to one side of an axial direction;
• and
• an index plate with which the stopper section comes into contact when the body section is pressed to one side in the axial direction,
• wherein the index plate comprises a plurality of plate sections having different thicknesses, a plurality of the plate sections being disposed on a same circumference,
• wherein the index plate has a penetrating section configured not to come into contact with the stopper section in the axial direction when the body section is moved by a maximum amount in the axial direction, and
• wherein the index plate is configured to rotate together with the body section in response to a rotation of the rotary unit in a state in which the stopper section is engaged in a circumferential direction with a wall surface, which is positioned at an end of the penetrating section in the circumferential direction.