An ultra-high pressure generator for generating pressure over 600 MPa, comprising: an intensifier that discharges pressurized fluid and includes a double-acting drive cylinder formed to have a first chamber and a second chamber which are delimited by a piston driven by a working medium, a high pressure cylinder (451,452) that discharges the pressurized fluid, and a plunger (461,462) that reciprocates with the piston in the high pressure cylinder (451,452); a closed-circuit working medium pump that has a first port and a second port as suction/discharge ports for the working medium, and sucks/discharges the working medium from/to the first chamber and the second chamber respectively via the first port and the second port to drive the intensifier; a drive source that drives the closed-circuit working medium pump; a first working medium channel that communicates the first chamber with the first port; and a second working medium channel that communicates the second chamber with the second port; characterized in that the ultra-high pressure generator further comprises: a working medium tank that stores the working medium, said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium; a supply circuit that is provided between the first working medium channel and the second working medium channel, and that communicates with the working medium tank; a selection circuit that is provided between the first working medium channel and the second working medium channel; and a recovery circuit that communicates the selection circuit with the working medium tank. an intensifier that discharges pressurized fluid and includes a double-acting drive cylinder formed to have a first chamber and a second chamber which are delimited by a piston driven by a working medium, a high pressure cylinder (451,452) that discharges the pressurized fluid, and a plunger (461,462) that reciprocates with the piston in the high pressure cylinder (451,452); a closed-circuit working medium pump that has a first port and a second port as suction/discharge ports for the working medium, and sucks/discharges the working medium from/to the first chamber and the second chamber respectively via the first port and the second port to drive the intensifier; a drive source that drives the closed-circuit working medium pump; a first working medium channel that communicates the first chamber with the first port; and a second working medium channel that communicates the second chamber with the second port; characterized in that the ultra-high pressure generator further comprises: a working medium tank that stores the working medium, said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium; a supply circuit that is provided between the first working medium channel and the second working medium channel, and that communicates with the working medium tank; a selection circuit that is provided between the first working medium channel and the second working medium channel; and a recovery circuit that communicates the selection circuit with the working medium tank. a working medium tank that stores the working medium, said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium; a supply circuit that is provided between the first working medium channel and the second working medium channel, and that communicates with the working medium tank; a selection circuit that is provided between the first working medium channel and the second working medium channel; and a recovery circuit that communicates the selection circuit with the working medium tank.