A Rankine cycle system, wherein a feed-forward value (NFF) is calculated based on the flow rate (Q) of a vapor-phase acting medium at the outlet of a evaporator (12) and a target pressure (PO) so that the pressure (P) of a vapor-phase acting medium at the inlet of an expander (13) agrees with a target pressure (PO), a feed-back value (NFB) is calculated by multiplying a deviation between the pressure (P) of a vapor-phase acting medium at the inlet of the expander (13) and a target pressure (PO) by feed-back gain (kp) calculated based on the flow rate (Q) of the above vapor-phase acting medium, and the rotation speed of the expander (13) is controlled based on the added/subtracted value of the feed-forward value (NFF) and the feed-back value (NFB). Accordingly, the pressure of a vapor-phase acting medium at the inlet of an expander can be controlled to a target pressure accurately without changing the supply amount of a liquid-phase acting medium to an evaporator.