side cylinders 2 a and 2 b to have grooves individually moreover the groove of the cylinder 2 d is made to have a shorter axial length than that of the groove of the cylinder 2 c as a result the degree of freedom of the piston 3 d has a smaller degree of freedom than that of the piston 3 c thus all the cylinders 2 a and 2 b 2 c and 2 d are formed as shown in fig3 such that the grooves 2 a 3 and 2 b 3 for enlarging the cylinder diameters are formed close to the bottoms 2 a 1 and 2 b 1 moreover the groove 2 b 3 of the cylinder 2 b 2 d is made shorter than the groove 2 a 3 of the other cylinder 2 a 2 c from the bottoms 2 a 1 and 2 b 1 of the cylinders 2 a and 2 b 2 c and 2 d so that the piston 3 b 3 d inserted into the cylinder 2 b 2 d and having groove 2 b 3 has a smaller allowable inclination angle with respect to the caliper 2 than that of the piston 3 a 3 c inserted into the other cylinder 2 a 2 c as a result the pistons 3 a and 3 b 3 c and 3 d are adjusted in the allowable inclination angles by the axial lengths of the grooves 2 a 3 and 2 b 3 formed in the cylinders 2 a and 2 b 2 c and 2 d moreover the piston 3 b 3.