In an acoustic resonator, an actuator allows a piston to reciprocate axially at very small amplitude at high speed. Owing to pressure fluctuation in the acoustic resonator involved by reciprocal motion of the piston, fluid is sucked into and discharged from the acoustic resonator via a valve device at the top end of the acoustic resonator. The acoustic resonator is covered with a gas guide with a space. The valve device is cooled by a fan at the top end of the gas guide.