Prior to bringing a fluid machine to the condition of spinning reserve for practicing a phase modifier operation or for producing a so-called standby condition which permits immediate switch-over to a power-generating or pumping-up operation, the water level in a runner chamber is forced down by supplying compressed air, and then the runner is driven to rotate in compressed air atmosphere. In this process, the water in a casing is gradually discharged at a rate which is commensurate with the rate of supply of the compressed air so as to thereby prevent leaking of the water in the casing from a guide vane.