A steam power cycle system (10, 20), which comprises: an evaporator (11, 21) configured to cause a working fluid of non-azeotropic mixture to make heat exchange with a predetermined high-temperature fluid and evaporate at least part of said working fluid; a gas-liquid separator (12, 22) configured to separate the working fluid having a high temperature and obtained by said evaporator into a gas phase substance and a liquid phase substance; an expander (13, 23) configured to receive the gas phase substance of said working fluid as introduced to convert heat energy held by the working fluid into power; a condenser (14, 15, 24, 25) configured to cause the working fluid in a gas phase from said expander and the working fluid in a liquid phase from said gas-liquid separator together to make heat exchange with a predetermined low-temperature fluid and condense the gas phase substance; and a pump (16, 26) configured to pump the working fluid from said condenser toward said evaporator, wherein: said condenser is a plurality of condensers (14, 15, 24, 25), each of said condensers having a flow channel on a working fluid side, which is connected in series and each of said condensers having a flow channel on a low-temperature fluid side, which is connected in series, so as to provide a flow channel design in which the low-temperature fluid passes through the respective condensers in reverse order to an order in which the working fluid passes through the condensers; further comprising a channel for drawing a part of the working fluid in the gas phase from an outlet of said expander to respective stages where the working fluid in the gas phase is joined together with working fluid that is discharged from a preceding condenser of adjacent condensers; and wherein a first condenser of the plurality of condensers is provided at a most anterior stage and at a position closest to the outlet of said expander on the channel for the working fluid, said first condenser configured to condense a further part of working fluid in the gas phase from said expander, and wherein a further condenser of the plurality of condensers is configured to condense the working fluid in the gas phase as joined together at the respective stages. an evaporator (11, 21) configured to cause a working fluid of non-azeotropic mixture to make heat exchange with a predetermined high-temperature fluid and evaporate at least part of said working fluid; a gas-liquid separator (12, 22) configured to separate the working fluid having a high temperature and obtained by said evaporator into a gas phase substance and a liquid phase substance; an expander (13, 23) configured to receive the gas phase substance of said working fluid as introduced to convert heat energy held by the working fluid into power; a condenser (14, 15, 24, 25) configured to cause the working fluid in a gas phase from said expander and the working fluid in a liquid phase from said gas-liquid separator together to make heat exchange with a predetermined low-temperature fluid and condense the gas phase substance; and a pump (16, 26) configured to pump the working fluid from said condenser toward said evaporator, wherein: said condenser is a plurality of condensers (14, 15, 24, 25), each of said condensers having a flow channel on a working fluid side, which is connected in series and each of said condensers having a flow channel on a low-temperature fluid side, which is connected in series, so as to provide a flow channel design in which the low-temperature fluid passes through the respective condensers in reverse order to an order in which the working fluid passes through the condensers; further comprising a channel for drawing a part of the working fluid in the gas phase from an outlet of said expander to respective stages where the working fluid in the gas phase is joined together with working fluid that is discharged from a preceding condenser of adjacent condensers; and wherein a first condenser of the plurality of condensers is provided at a most anterior stage and at a position closest to the outlet of said expander on the channel for the working fluid, said first condenser configured to condense a further part of working fluid in the gas phase from said expander, and wherein a further condenser of the plurality of condensers is configured to condense the working fluid in the gas phase as joined together at the respective stages. said condenser is a plurality of condensers (14, 15, 24, 25), each of said condensers having a flow channel on a working fluid side, which is connected in series and each of said condensers having a flow channel on a low-temperature fluid side, which is connected in series, so as to provide a flow channel design in which the low-temperature fluid passes through the respective condensers in reverse order to an order in which the working fluid passes through the condensers; further comprising a channel for drawing a part of the working fluid in the gas phase from an outlet of said expander to respective stages where the working fluid in the gas phase is joined together with working fluid that is discharged from a preceding condenser of adjacent condensers; and wherein a first condenser of the plurality of condensers is provided at a most anterior stage and at a position closest to the outlet of said expander on the channel for the working fluid, said first condenser configured to condense a further part of working fluid in the gas phase from said expander, and wherein a further condenser of the plurality of condensers is configured to condense the working fluid in the gas phase as joined together at the respective stages.