PURPOSE: To prevent the sudden rise of input at the time of starting and reduce a bad influence to an electric power system by opening guide vanes while raising the number of revolutions at a revolution which is lower than a steady revolution and does not cause back flow.

CONSTITUTION: In starting pump, after an underwater no-discharge operating condition has been realized at the minimum starring revolution N2, the opening of guide vanes is started to open to raise the number of revolutions to N1, and guide vanes are opened to a stable starting opening GVO1. And, since the pump is in a steady stable operating condition at the time when the number of revolutions is N1 and the opening and the opening of guide vanes is in the stable starting opening GVO1, and after that the pump is shifted to a prescribed revolution N0, guide vane opening GVO0 by the ordinary speed and opening control method for the variable speed pump.