PURPOSE: To change the boosting ratio from the initial braking by arranging an operating rod of double construction so that a second rod thereof receives the pedal operation input and then, a second rod receives it together with the second rod.

CONSTITUTION: When a pedal 19 is depressed, a spool 43 is pushed to the left by way of a second rod 63 of an operating rod 31 to reduce a throttle 48 so that a hydraulic pressure is generated in a groove 29a on the upstream of the throttle. The hydraulic pressure reaches a boost chamber 45 to move a power piston 29 to the left. At the same time, a first rod 64 moves together with the piston 29 through a disc spring 60. The output of the piston drives a master cylinder. As the rod 63 pushed, the piston further moves while the movement of the piston becomes larger than that of the rod 63 corresponding to the rise in the hydraulic pressure. The hydraulic pressure suppresses the movement of the rod 64 whereby the disch spring 60 is compressed. A stepped section 51a gets into contact with a shoulder section 69. Thereafter, both the rods 63 and 64 push the spool 43 together to change the boosting ratio.