A hydraulic circuit suitable for use in a hydraulic operating circuit of a power shovel includes a hydraulic cylinder for driving a working component and a hydraulic change-over valve for selectively supplying the cylinder with a discharge oil pressure of a hydraulic pump driven from an engine. The hydraulic circuit prevents cavitation in an oil chamber of the cylinder and includes a pair of first signal receiving portions provided in the hydraulic change-over valve for moving the spool thereof in the forward and reverse directions away from a netural position, respectively, and a second signal receiving portion for pulling back the spool toward said neutral position. A pressure detector produces a signal commensurate with the pressure in an oil chamber of the cylinder opposing an oil chamber where vacuum pressure is developed by the weight of the working component and a signalling device produces a signal indicating the extent of pulling back the spool toward the neutral position on receipt of the results of detection by the pressure detector, thereby supplying the signal to the second signal receiving portion of the hydraulic change-over valve.