A measurement control section (4) for controlling operation of measurement, by a time measuring section (11), of the propagation time of ultrasonic wave operates based on a high rate clock supplied from a ceramic oscillation circuit (16) during time measuring operation. Every time when time measuring operation by the time measuring section ends, a clock control section (18) stops the high rate clock supplied from the ceramic oscillation circuit (16), and the clock control section (18) controls a waiting time, by a low rate clock supplied from a quartz oscillation circuit (15), after the end of measurement of propagation time until the start of the next measurement.