A driving force transmission mechanism of a clamp apparatus includes first link arms, which are pivotally supported for rotation on ends of first and second piston rods, and second link arms, which connect other ends of the first link arms to respective ends on one end side of first and second clamp arms. Further, first rollers, which are provided on the one end of the first link arms, are guided in a horizontal direction along guide grooves of first guide members, and second rollers, which are provided on the other ends of the first link arms and the second link arms, are guided in a vertical direction along guide grooves of second guide members.