PURPOSE: To shorten a time delay required for a pressure rise of fluid in the second fluid pressure chamber, in the case of failure in the first fluid pressure chamber.

CONSTITUTION: A stop part 38 stopping advancing a small diametric piston 34 in a small diametric cylinder 33 is provided in the small diametric cylinder, to arrange between the small diametric piston and a cylinder front end wall in a free length condition an elastic material 40 having elastic force in a compressive direction further having elastic force stronger than a small diametric piston spring 42 of energizing the small diametric piston in a forwardly advancing direction. In the case of generating failure in the first fluid pressure chamber 9, after an ineffective stroke, corresponding to a delivery amount of the first fluid pressure chamber until a main piston 4 comes into mechanical contact with a floating piston 5, is performed, immediately the elastic material is while compressed, so as to generate a fluid pressure in the second fluid pressure chamber 11.