A Rankine cycle device in the present disclosure includes an evaporator as a heater, an expander, a cooler, a first temperature sensor, a second temperature sensor, and a control device. The first temperature sensor detects a temperature of the working fluid flowing from an outlet of the heater to an inlet of the expander in the circuit of the working fluid. The second temperature sensor detects a temperature of the working fluid flowing from an outlet of the expander to an inlet of the cooler. The controller controls a number of rotation of the expander based on a difference between a detected temperature of the first temperature sensor and a detected temperature of the second temperature sensor.