An ultrasonic element generates an ultrasonic wave and converts an input ultrasonic wave into an electric signal. A transmission circuit outputs a drive signal to the ultrasonic element. A comparator circuit outputs a first detection signal when the electric signal becomes larger than a threshold value and outputs a second detection signal when the electric signal becomes smaller than the threshold value. An arithmetic circuit computes a distance of a propagation path of the ultrasonic wave based on a time difference between an output timing, at which the ultrasonic element outputs of the ultrasonic wave, and a liquid level timing, at which the comparator circuit outputs the first detection signal, and based on a propagation speed of the ultrasonic wave. A storage unit stores a time difference between the first detection signal and the second detection signal. The transmission circuit increases the drive signal, as the time difference decreases.