A scroll expander includes a housing, a main port, a high-pressure chamber, an operation chamber, a low-pressure chamber, a subport, a valve portion, an actuator, and a drive shaft. The high-pressure and low-pressure chambers are separated by a partition wall. The main port draws working fluid in the high-pressure chamber into the operation chamber. The subport introduces the working fluid from the high-pressure chamber to the operation chamber so that the volume of the working fluid in the operation chamber is variable. The valve portion selectively opens and closes the subport. The actuator is arranged in the low-pressure chamber. The drive shaft extends through the partition wall and connects the actuator and the valve portion with each other. When the drive shaft is actuated by the actuator, the subport is selectively opened and closed by the valve portion.