To provide a thermoacoustic engine for easily and securely providing desired heat efficiency and an output.

The termoacoustic engine 20 includes an air column pipe 21 with a working fluid as mixed gas of helium and argon encapsulated, and a heat accumulator 150 arranged inside the air column pipe 21, and generates the thermoacoustic self-exited oscillation of the working fluid by forming a temperature gradient between both ends of the heat accumulator 150. The engine is provided with a fluid separating device 30 for changing a mixture ratio of the helium and the argon in the working fluid, including a pump 28, a working fluid introducing valve 29, a working fluid recovery pipe L4, and a separation membrane 32, first and second supply valves 33, 34, and fluid introducing pipes L5, L6.