PURPOSE: To provide a safety system against pump run-out, which comprises an outlet valve disposed at a portion of outlet piping of pump, said outlet valve being arranged such that it is opened and closed depending on the pressure of fluid at the outlet of pump, whereby pump can be protected from run-out positively and with high responsibility under any circumstances.

CONSTITUTION: At a portion of outlet piping 2 of pump 1, there is provided an outlet valve 3 which is opened and closed depending on the pressure at the outlet of pump 1. In case, for instance, that pressure in the outlet piping 2 is high, movable piston 43 is shifted upward and driving fluid is fed to the lower portion of driving piston 22, whereby outlet valve 3 is opened. On the other hand, in case that pressure in the outlet piping 2 is low, piston 43 is shifted downward and fluid is discharged from the lower portion of driving piston 22, whereby outlet valve 3 is closed. Thus, because of the arrangement that opening and closing of the outlet valve 3 is controlled by pressure at the outlet of pump, it is free from electric troubles and pump can be protected from run-out positively and with high responsibility under any circumstances.