To add a function of detecting a temperature to an ultrasonic flow sensor without making the structure of the ultrasonic flow sensor complicated. A sensor portion of the ultrasonic flow sensor includes an acoustic coupling member. The sensor portion includes a heat transfer member having a heat detecting surface for detecting heat of piping. The temperature of heat transferred in the heat transfer member via the heat detecting surface is detected by a temperature detecting device. The heat transfer member restricts an amount of collapse of the acoustic coupling member in a radial direction of the piping when the sensor portion is fixed to the piping.