A maintenance method for a power generation system comprising: a medium circuit through which a medium with a boiling point lower than that of water is circulated; a circulation pump configured to have the medium circulating through the medium circuit; an evaporator configured to heat the medium using heat of an external source so as to evaporate the medium; a condenser configured to condense the medium discharged from the expander; and a generator configured to be driven by the expander to generate power; wherein the medium circuit branches into a plurality of branched pipes (21a, 21b, 21c) between the downstream side of the evaporator and the upstream side of the condenser, the branched pipes each have a maintenance target device, which is an expander configured to be driven using the medium evaporated by the evaporator, a first on-off valve, a second on-off valve, a port, and a third on-off valve, each first on-off valve is installed on the medium circuit at an upstream side of the maintenance target device including at least one of the circulation pump, the evaporator, the expander, and the condenser, and is capable of shutting off the flow of the medium through the medium circuit, each second on-off valve is installed on the medium circuit at a downstream side of the maintenance target device, and is capable of shutting off the flow of the medium through the medium circuit, each port is configured to be communicable with the medium circuit between the first on-off valve and the second on-off valve, each third on-off valve is installed on the port, and a control unit is provided to control the operation of the circulation pump, and the operation of the first on-off valve, second on-off valve and third on-off valve, characterized in that the control unit is configured to select one maintenance target device to be maintained in accordance with an operation state of each of the expanders, the maintenance method comprising: stopping the circulation pump corresponding to the maintenance target device and closing the first on-off valve on the upstream side of the maintenance target device and the second on-off valve on the downstream side of the maintenance target device; opening the third on-off valve and recovering the medium between the first on-off valve and the second on-off valve when recovering the medium from the port; closing the third on-off valve after a section between the first on-off valve and the second on-off valve is vacuumed after the maintenance of the maintenance target device is completed; and opening the third on-off valve, and filling the section between the first on-off valve and the second on-off valve with the medium when filling the medium from the port. a medium circuit through which a medium with a boiling point lower than that of water is circulated; a circulation pump configured to have the medium circulating through the medium circuit; an evaporator configured to heat the medium using heat of an external source so as to evaporate the medium; a condenser configured to condense the medium discharged from the expander; and a generator configured to be driven by the expander to generate power; wherein the medium circuit branches into a plurality of branched pipes (21a, 21b, 21c) between the downstream side of the evaporator and the upstream side of the condenser, the branched pipes each have a maintenance target device, which is an expander configured to be driven using the medium evaporated by the evaporator, a first on-off valve, a second on-off valve, a port, and a third on-off valve, each first on-off valve is installed on the medium circuit at an upstream side of the maintenance target device including at least one of the circulation pump, the evaporator, the expander, and the condenser, and is capable of shutting off the flow of the medium through the medium circuit, each second on-off valve is installed on the medium circuit at a downstream side of the maintenance target device, and is capable of shutting off the flow of the medium through the medium circuit, each port is configured to be communicable with the medium circuit between the first on-off valve and the second on-off valve, each third on-off valve is installed on the port, and a control unit is provided to control the operation of the circulation pump, and the operation of the first on-off valve, second on-off valve and third on-off valve, stopping the circulation pump corresponding to the maintenance target device and closing the first on-off valve on the upstream side of the maintenance target device and the second on-off valve on the downstream side of the maintenance target device; opening the third on-off valve and recovering the medium between the first on-off valve and the second on-off valve when recovering the medium from the port; closing the third on-off valve after a section between the first on-off valve and the second on-off valve is vacuumed after the maintenance of the maintenance target device is completed; and opening the third on-off valve, and filling the section between the first on-off valve and the second on-off valve with the medium when filling the medium from the port.