To provide a stable and highly-reliable booster water supply system saving energy, by preventing a decrease in water supply pressure, hunting, etc. from occurring in a zone or local area of a partial story, and preventing an excessive operation of a lower-story pump.

This booster water supply system includes a means which determines a usage state of water in a high-story zone by a frequency of a variable-speed driving means, which has a parameter for this determination provided and stored in a storage portion, and which transmits an actual frequency in the story zone as a lower-story zone operational-status signal. A means for comparing parameters for determining the quantity of water used on the side of the high-story zone is provided on the side of a lower story receiving the signal. A stable and highly-reliable interconnected operation section between respective booster water supply sections is constructed by determining whether to perform a variable-speed operation or a maximum speed-fixed one.