in general the predetermined period of time is determined to be approximately the half of the lowering time or rising time that is at time t 2 when the predetermined period of time has passed the pressure p inside the second cylinder 4 and the pressure p inside the first cylinder 2 are substantially equal at time t 2 that is after continuation of the open state of the communicating valve v 5 for the predetermined period of time the communicating valve v 5 is closed by the control part further at time t 2 the first intake valve v 1 is opened to allow high pressure helium gas to be supplied from the high pressure side h of the compressor 8 into the first cylinder 2 via the first supply pipe 61 and the first supply and outlet common pipe 65 so that the pressure p inside the first cylinder 2 is caused to be high h further at time t 3 when a predetermined period of time has passed since time t 2 the first intake valve v 1 is closed likewise at time t 2 the second outlet valve v 4 is opened to allow helium gas inside the second cylinder 4 to be discharged to the low pressure side l of the compressor 8 via the second supply and outlet common pipe 66 and the second outlet pipe 64 so that the pressure p inside the second cylinder 4 is caused to be low l further at time t 3 when the predetermined period of time has passed since time t 2 the second outlet valve.