There is provided a steam power cycle system that permits to perform an appropriate heat exchange between a working fluid that is a non-azeotropic mixture and a heat source, to enhance the performance of the whole system. More specifically, a plurality of condensers are provided so as to be connected to each other in series, and the working fluid in a gas phase from the expander is introduced into the respective condensers. Consequently, the ratio of a low boiling point substance of the working fluid becomes higher toward the posterior condenser, it is possible to make the condensation temperature of the working fluid lower than that of the anterior condenser. It is therefore possible to make the temperature of the working fluid possibly close to the temperature of the low-temperature fluid, thus permitting an effective use of the difference in temperature of the heat source.