PURPOSE: To remove useless power consumption by inputting measured propagation time and reflected wave energy to execute fuzzy inference, determining the adjusting width of transmission power and adjusting the transmission power in accordance with the adjusting width.

CONSTITUTION: When a projected physical variable is reflected by a moving object and returned, the propagation time from transmission up to reception is measured, a distance up to the moving object is measured and the whole energy of received waves, i.e. reflected waves, is found out. The integrated value of an integrating amplifier circuit 9 is inputted to a fuzzy inference circuit 10 as the whole energy of the reflected waves and the propagation time is also inputted from a propagation time counting circuit 7 to the circuit 10. The circuit 10 executes the fuzzy inference of the inputted propagation time and reflected wave energy and determines the execution width of transmission power and a power adjusting circuit 11 adjusts the transmission power of an ultrasonic wave generating circuit 2 in accordance with the determined adjusting width. Consequently, power can be saved while keeping stable operation.