An ultra-high pressure generator for generating pressure over 600 MPa, comprising: an intensifier that discharges pressurized fluid (F2) 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 (F1), a high pressure cylinder that discharges the pressurized fluid (F2), and a plunger that reciprocates with the piston in the high pressure cylinder ; a closed-circuit working medium pump that has a first port and a second port as suction/discharge ports for the working medium (F1), 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 (F1), said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium (F1); 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 (F2) 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 (F1), a high pressure cylinder that discharges the pressurized fluid (F2), and a plunger that reciprocates with the piston in the high pressure cylinder ; a closed-circuit working medium pump that has a first port and a second port as suction/discharge ports for the working medium (F1), 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 (F1), said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium (F1); 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 (F1), said working medium tank being applied with an internal pressure and being adapted to regulate the total amount of the working medium (F1); 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 . <EOS>