A fluid pressure-feed device that feeds a fluid under pressure to a fluid passage, the device comprising: a main tank configured to be filled with the fluid; a main follower plate that applies pressurizing force to the fluid inside the main tank; a sub tank into which the fluid forced out from the main tank flows, the sub tank being provided either integrally with the main follower plate or being connected to said plate by a pipe such that the sub tank moves integrally with upward and downward movements of the main follower plate; a pump that feeds the fluid inside the sub tank under pressure toward the fluid passage; a sub follower plate that is configured to apply pressurizing force to the fluid inside the sub tank, the pressurizing force being used to force out the fluid toward the pump during a replacement operation of the main tank; sub cylinders attached to an upper portion of the pump with piston rods extending on top of the sub cylinders, respectively, upper ends of the piston rods being attached to a tie bar such that when the tie bar moves upward and downward the main follower plate, the sub tank, the sub follower plate, and the pump move upward and downward; and main cylinders with piston rods extending on top, upper ends of the piston rods being attached to the tie bar, such that the main cylinders are adapted to move the tie bar upward and downward wherein an upper surface of the main follower plate and the tie bar are connected with each other by tie rods; wherein when the sub cylinders are actuated, the pump and the sub follower plate move upward and downward with respect to the sub tank; and wherein the device is configured such that, in a state where a sufficient amount of the fluid remains in the main tank, the fluid extruded from the main tank flows into the sub tank, and the fluid inside the sub tank is then fed under pressure by the pump towards the fluid passage. a main tank configured to be filled with the fluid; a main follower plate that applies pressurizing force to the fluid inside the main tank; a sub tank into which the fluid forced out from the main tank flows, the sub tank being provided either integrally with the main follower plate or being connected to said plate by a pipe such that the sub tank moves integrally with upward and downward movements of the main follower plate; a pump that feeds the fluid inside the sub tank under pressure toward the fluid passage; a sub follower plate that is configured to apply pressurizing force to the fluid inside the sub tank, the pressurizing force being used to force out the fluid toward the pump during a replacement operation of the main tank; sub cylinders attached to an upper portion of the pump with piston rods extending on top of the sub cylinders, respectively, upper ends of the piston rods being attached to a tie bar such that when the tie bar moves upward and downward the main follower plate, the sub tank, the sub follower plate, and the pump move upward and downward; and main cylinders with piston rods extending on top, upper ends of the piston rods being attached to the tie bar, such that the main cylinders are adapted to move the tie bar upward and downward wherein an upper surface of the main follower plate and the tie bar are connected with each other by tie rods; wherein when the sub cylinders are actuated, the pump and the sub follower plate move upward and downward with respect to the sub tank; and wherein the device is configured such that, in a state where a sufficient amount of the fluid remains in the main tank, the fluid extruded from the main tank flows into the sub tank, and the fluid inside the sub tank is then fed under pressure by the pump towards the fluid passage. a sub follower plate that is configured to apply pressurizing force to the fluid inside the sub tank, the pressurizing force being used to force out the fluid toward the pump during a replacement operation of the main tank; sub cylinders attached to an upper portion of the pump with piston rods extending on top of the sub cylinders, respectively, upper ends of the piston rods being attached to a tie bar such that when the tie bar moves upward and downward the main follower plate, the sub tank, the sub follower plate, and the pump move upward and downward; and main cylinders with piston rods extending on top, upper ends of the piston rods being attached to the tie bar, such that the main cylinders are adapted to move the tie bar upward and downward wherein an upper surface of the main follower plate and the tie bar are connected with each other by tie rods; wherein when the sub cylinders are actuated, the pump and the sub follower plate move upward and downward with respect to the sub tank; and wherein the device is configured such that, in a state where a sufficient amount of the fluid remains in the main tank, the fluid extruded from the main tank flows into the sub tank, and the fluid inside the sub tank is then fed under pressure by the pump towards the fluid passage.