The amplifying gain is controlled by means of a data reduction equipment on the basis of the control signal input into the amplifier for amplifying the received signal propagated through the fluid and received in the ultrasonic transducer in the receiving side. The data reduction equipment includes an analog-digital converter, a processor, and a memory, wherein converted into digital data are waveform data including a peak value of each of the received signals, and the obtained digital data are stored in the memory. The processor screens the peak values stored in the memory to obtain a screened peak value achieved on the fluid including no impurities such as bubbles or solid particles, compares the obtained peak value with the preset peak value set preliminary on the processor, sends a control signal to the gain control terminal of the amplifier on the basis of the result of the comparison, and controls the peak value included in the output signal of the amplifier so as to take the substantially equal value as that of the preset value of the peak.