A Rankine cycle apparatus (1A - 1F) comprising: a main circuit formed by an expander , a condenser , a pump , and an evaporator that are circularly connected in this order; a heat exchange portion (HX) located in the main circuit at a position between an outlet of the expander and an inlet of the pump ; a bypass flow path branching from the main circuit at a position between an outlet of the evaporator and an inlet of the expander and joining to the main circuit at a position between the outlet of the expander and an inlet of the heat exchange portion (HX); a flow rate-adjusting mechanism that adjusts a flow rate of a working fluid in the bypass flow path ; a pair of temperature sensors (7A - 7F) that detects temperatures of the working fluid at two positions spaced from each other in a flow direction of the working fluid in a portion of the main circuit between a junction point at which the bypass flow path joins to the main circuit and an inlet of the evaporator , and a controller that controls the flow rate-adjusting mechanism , wherein the two positions are arranged such that when the working fluid flowing into the heat exchange portion (HX) is a superheated vapor, a difference between the temperature of the working fluid at one of the two positions and the temperature of the working fluid at the other of the two positions is equal to or larger than a predetermined value, characterized in that the controller is configured to control the flow rate-adjusting mechanism so that the flow rate of the working fluid in the bypass flow path is reduced when a difference between two temperatures detected by the pair of temperature sensors (7A - 7F) exceeds a first threshold. a main circuit formed by an expander , a condenser , a pump , and an evaporator that are circularly connected in this order; a heat exchange portion (HX) located in the main circuit at a position between an outlet of the expander and an inlet of the pump ; a bypass flow path branching from the main circuit at a position between an outlet of the evaporator and an inlet of the expander and joining to the main circuit at a position between the outlet of the expander and an inlet of the heat exchange portion (HX); a flow rate-adjusting mechanism that adjusts a flow rate of a working fluid in the bypass flow path ; a pair of temperature sensors (7A - 7F) that detects temperatures of the working fluid at two positions spaced from each other in a flow direction of the working fluid in a portion of the main circuit between a junction point at which the bypass flow path joins to the main circuit and an inlet of the evaporator , and a controller that controls the flow rate-adjusting mechanism , wherein the two positions are arranged such that when the working fluid flowing into the heat exchange portion (HX) is a superheated vapor, a difference between the temperature of the working fluid at one of the two positions and the temperature of the working fluid at the other of the two positions is equal to or larger than a predetermined value, characterized in that the controller is configured to control the flow rate-adjusting mechanism so that the flow rate of the working fluid in the bypass flow path is reduced when a difference between two temperatures detected by the pair of temperature sensors (7A - 7F) exceeds a first threshold. <EOS>