WE CLAIM:
Claim 1. A pressure booster (10, 10A, 10B), comprising:
a pressure boosting chamber (32);
a first drive chamber (34) disposed on one end side of the pressure boosting chamber (32);
a second drive chamber (36) disposed on another end side of the pressure boosting chamber (32);
a piston rod (42) configured to penetrate through the pressure boosting chamber (32) and extend to the first drive chamber (34) and the second drive chamber (36);
a pressure boosting piston (44) which, by being connected to the piston rod (42) inside the pressure boosting chamber (32), is configured to partition the pressure boosting chamber (32) into a first pressure boosting chamber (32a) on a side of the first drive chamber (34), and a second pressure boosting chamber (32b) on a side of the second drive chamber (36);
a first drive piston (46) which, by being connected to one end of the piston rod (42) inside the first drive chamber (34), is configured to partition the first drive chamber (34) into a first pressurizing chamber (34a) on a side of the first pressure boosting chamber (32a), and a second pressurizing chamber (34b) remote from the first pressure boosting chamber (32a);
a second drive piston (48) which, by being connected to another end of the piston rod (42) inside the second drive chamber (36), is configured to partition the second drive chamber (36) into a third pressurizing chamber (36a) on a

side of the second pressure boosting chamber (32b), and a fourth pressurizing chamber (36b) remote from the second pressure boosting chamber (32b);
a fluid supplying mechanism (52) configured to supply a fluid to at least one of the first pressure boosting chamber (32a) and the second pressure boosting chamber (32b);
a first discharge return mechanism (22) configured to supply the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b), or to supply the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a); and
a second discharge return mechanism (26) configured to supply the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b), or to supply the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a).
Claim 2. The pressure booster (10, 10A, 10B) according to claim 1, wherein:
in a case that the fluid is supplied from the fluid supplying mechanism (52) to the first pressure boosting chamber (32a), at least, the first discharge return mechanism (22) supplies the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b), or the second discharge return mechanism (26) supplies the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a);
whereas, in a case that the fluid is supplied from the fluid supplying mechanism (52) to the second pressure boosting chamber (32b), at least, the second discharge

return mechanism (26) supplies the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b), or the first discharge return mechanism (22) supplies the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a).
Claim 3. The pressure booster (10) according to claim 2, wherein:
in a case that the fluid is supplied from the fluid supplying mechanism (52) to the first pressure boosting chamber (32a), the first discharge return mechanism (22) supplies the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b), based on a difference, on the first drive piston (46), between a pressure receiving area on a side of the first pressurizing chamber (34a) and a pressure receiving area on a side of the second pressurizing chamber (34b), and the second discharge return mechanism (26) supplies the fluid to the third pressurizing chamber (36a) together with discharging the fluid from the fourth pressurizing chamber (36b);
whereas, in a case that the fluid is supplied from the fluid supplying mechanism (52) to the second pressure boosting chamber (32b), the first discharge return mechanism (22) supplies the fluid to the first pressurizing chamber (34a) together with discharging the fluid from the second pressurizing chamber (34b), and the second discharge return mechanism (26) supplies the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b), based on a difference, on the second drive

piston (48), between a pressure receiving area on a side of the third pressurizing chamber (36a) and a pressure receiving area on a side of the fourth pressurizing chamber (36b).
Claim 4. The pressure booster (10) according to claim
3, wherein:
the first discharge return mechanism (22) is configured to include a solenoid valve (22a, 22b) which is configured to supply the fluid supplied from exterior to the fluid supplying mechanism (52) to the first pressurizing chamber (34a) together with discharging the fluid of the second pressurizing chamber (34b) to the exterior, and on the other hand, is configured to supply the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b); and
the second discharge return mechanism (26) is configured to include a solenoid valve (26a, 26b) which is configured to supply the fluid supplied from the exterior to the fluid supplying mechanism (52) to the third pressurizing chamber (36a) together with discharging the fluid of the fourth pressurizing chamber (36b) to the exterior, and on the other hand, is configured to supply the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b).
Claim 5. The pressure booster (10) according to claim
4, wherein:
the first discharge return mechanism (22) is configured to include a first solenoid valve (22a) connected to the

first pressurizing chamber (34a), a second solenoid valve (22b) connected to the second pressurizing chamber (34b), and a first discharge return flow passage (70) connected with the first solenoid valve (22a) and the second solenoid valve (22b);
at a first position of the first solenoid valve (22a) and the second solenoid valve (22b), the first pressurizing chamber (34a) and the second pressurizing chamber (34b) communicate with each other through the first discharge return flow passage (70);
at a second position of the first solenoid valve (22a) and the second solenoid valve (22b), the first pressurizing chamber (34a) communicates with the fluid supplying mechanism (52), and the second pressurizing chamber (34b) communicates with the exterior;
the second discharge return mechanism (26) is configured to include a third solenoid valve (26a) connected to the third pressurizing chamber (36a), a fourth solenoid valve (26b) connected to the fourth pressurizing chamber (36b), and a second discharge return flow passage (80) connected with the third solenoid valve (26a) and the fourth solenoid valve (26b);
at a first position of the third solenoid valve (26a) and the fourth solenoid valve (26b), the third pressurizing chamber (36a) and the fourth pressurizing chamber (36b) communicate with each other through the second discharge return flow passage (80); and
at a second position of the third solenoid valve (26a) and the fourth solenoid valve (26b), the third pressurizing chamber (36a) communicates with the fluid supplying

mechanism (52), and the fourth pressurizing chamber (36b) communicates with the exterior.
Claim 6. The pressure booster (10A) according to claim 2, wherein:
in a case that the fluid is supplied from the fluid supplying mechanism (52) to the first pressure boosting chamber (32a), the first discharge return mechanism (22) supplies the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b), together with the second discharge return mechanism (26) supplying the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a);
whereas, in a case that the fluid is supplied from the fluid supplying mechanism (52) to the second pressure boosting chamber (32b), the first discharge return mechanism (22) supplies the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a), together with the second discharge return mechanism (26) supplying the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b).
Claim 7. The pressure booster (10A) according to claim 6, wherein:
the first discharge return mechanism (22) is configured to include a three-way valve type fifth solenoid valve (120) which, in a first position, is configured to interrupt communication between the first pressurizing chamber (34a) and the second pressurizing chamber (34b), whereas in a

second position, is configured to allow communication between the first pressurizing chamber (34a) and the second pressurizing chamber (34b);
the fifth solenoid valve (120), by switching between a communication interrupted state and a communication allowed state, carries out supply of the fluid discharged from the first pressurizing chamber (34a) to the second pressurizing chamber (34b), or carries out supply of the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a);
the second discharge return mechanism (26) is configured to include a three-way valve type sixth solenoid valve (124) which, in a first position, is configured to allow communication between the third pressurizing chamber (36a) and the fourth pressurizing chamber (36b), whereas in a second position, is configured to interrupt communication between the third pressurizing chamber (36a) and the fourth pressurizing chamber (36b); and
the sixth solenoid valve (124), by switching between a communication interrupted state and a communication allowed state, carries out supply of the fluid discharged from the third pressurizing chamber (36a) to the fourth pressurizing chamber (36b), or carries out supply of the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a).
Claim 8. The pressure booster (10B) according to claim 2, wherein:
in a case that the fluid is supplied from the fluid supplying mechanism (52) to the first pressure boosting

chamber (32a), the first discharge return mechanism (22) discharges the fluid from the first pressurizing chamber (34a) together with supplying the fluid to the second pressurizing chamber (34b), and the second discharge return mechanism (26), while supplying a portion of the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a), discharges another portion of the fluid to exterior;
whereas, in a case that the fluid is supplied from the fluid supplying mechanism (52) to the second pressure boosting chamber (32b), the first discharge return mechanism (22), while supplying a portion of the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a), discharges another portion of the fluid to the exterior, and the second discharge return mechanism (26) discharges the fluid from the third pressurizing chamber (36a) together with supplying the fluid to the fourth pressurizing chamber (36b).
Claim 9. The pressure booster (10B) according to claim 8, wherein:
the first discharge return mechanism (22) is configured to include a seventh solenoid valve (140) which is configured to supply the fluid supplied from the exterior to the fluid supplying mechanism (52) to the second pressurizing chamber (34b) together with discharging the fluid of the first pressurizing chamber (34a) to the exterior, and on the other hand, while supplying a portion of the fluid discharged from the second pressurizing chamber (34b) to the first pressurizing chamber (34a), is configured

to discharge another portion of the fluid to the exterior; and
the second discharge return mechanism (26) is configured to include an eighth solenoid valve (146) which is configured to supply the fluid supplied from the exterior to the fluid supplying mechanism (52) to the fourth pressurizing chamber (36b) together with discharging the fluid of the third pressurizing chamber (36a) to the exterior, and on the other hand, while supplying a portion of the fluid discharged from the fourth pressurizing chamber (36b) to the third pressurizing chamber (36a), is configured to discharge another portion of the fluid to the exterior.
Claim 10. The pressure booster (10B) according to claim 9, wherein:
the first discharge return mechanism (22) is configured to include the seventh solenoid valve (140) of a four-way five-port solenoid valve, and a first check valve (142);
the seventh solenoid valve (140), in a first position, places the first pressurizing chamber (34a) in communication with the exterior together with placing the second pressurizing chamber (34b) in communication with the fluid supplying mechanism (52), whereas in a second position, places the second pressurizing chamber (34b) in communication with the exterior and in communication with the first pressurizing chamber (34a) via the first check valve (142);
the second discharge return mechanism (26) is configured to include the eighth solenoid valve (146) of a four-way five-port solenoid valve, and a second check valve

(148);
the eighth solenoid valve (146), in a first position, places the fourth pressurizing chamber (36b) in communication with the exterior and in communication with the third pressurizing chamber (36a) via the second check valve (148), whereas in a second position, places the third pressurizing chamber (36a) in communication with the exterior together with placing the fourth pressurizing chamber (36b) in communication with the fluid supplying mechanism (52).
Claim 11. The pressure booster (10, 10A, 10B) according to claim 1, further comprising:
a position detecting sensor (84a, 84b) configured to detect a position of the first drive piston (46) or the second drive piston (48);
wherein, based on a detection result of the position detecting sensor (84a, 84b), the first discharge return mechanism (22) and the second discharge return mechanism (26), respectively, carry out supply of the fluid discharged from one of the pressurizing chambers to another pressurizing chamber.
Claim 12. The pressure booster (10, 10A, 10B) according to claim 11, wherein the position detecting sensor (84a, 84b) comprises a first position detecting sensor (84a) configured to detect arrival of the first drive piston (46) or the second drive piston (48) at one end side of the first drive chamber (34) or the second drive chamber (36), and a second position detecting sensor (84b) configured to detect

arrival of the first drive piston (46) or the second drive piston (48) at another end side of the first drive chamber (34) or the second drive chamber (36).
Claim 13. The pressure booster (10, 10A, 10B) according to claim 11, wherein the position detecting sensor (84a, 84b) comprises a magnetic sensor configured to detect the position of the first drive piston (46) or the second drive piston (48) by detecting magnetism produced by a magnet (86) attached to the first drive piston (46) or the second drive piston (48).
Claim 14. The pressure booster (10A) according to claim 1, further comprising:
a pressure sensor (122, 126) configured to detect a pressure of the fluid discharged from one of the pressurizing chambers and supplied to another pressurizing chamber;
wherein, based on a detection result of the pressure sensor (122, 126), the first discharge return mechanism (22) and the second discharge return mechanism (26), respectively, stop supplying the fluid discharged from the one of the pressurizing chambers to the other pressurizing chamber.
Claim 15. The pressure booster (10) according to claim 1, wherein the fluid supplying mechanism (52) is configured to include a check valve (52c, 52d) configured to prevent back-flowing of the fluid from the first pressure boosting chamber (32a) and the second pressure boosting chamber

(32b).
Claim 16. The pressure booster (10) according to claim 15, further comprising:
a fluid output mechanism (58) configured to output to exterior the fluid that was boosted in pressure in the first pressure boosting chamber (32a) or the second pressure boosting chamber (32b);
wherein the fluid output mechanism (58) is configured to include a check valve (58c, 58d) configured to prevent back-flowing of the fluid into the first pressure boosting chamber (32a) and the second pressure boosting chamber (32b).
Claim 17. The pressure booster (10, 10A, 10B) according to claim 1, wherein a size of the first drive chamber (34) in a diametrical direction thereof and a size of the second drive chamber (36) in a diametrical direction thereof are smaller than a size of the pressure boosting chamber (32) in a diametrical direction thereof.
Claim 18. The pressure booster (10, 10A, 10B) according to claim 1, wherein:
a first cover member (18) is interposed between the first pressure boosting chamber (32a) and the first pressurizing chamber (34a);
a second cover member (20) is interposed between the second pressure boosting chamber (32b) and the third pressurizing chamber (36a);
a third cover member (38) is disposed on an end of the

second pressurizing chamber (34b) remote from the first cover member (18);
a fourth cover member (40) is disposed on an end of the fourth pressurizing chamber (36b) remote from the second cover member (20);
the first drive piston (46) is displaced inside the first drive chamber (34) without coming into contact with the first cover member (18) and the third cover member (38);
the second drive piston (48) is displaced inside the second drive chamber (36) without coming into contact with the second cover member (20) and the fourth cover member (40); and
the pressure boosting piston (44) is displaced inside the pressure boosting chamber (32) without coming into contact with the first cover member (18) and the second cover member (20).