To emit an acoustic pulse of an optimum frequency suitable to the Bragg's theorem to the atmosphere in a short time and to perform a highly precise temperature measurement by preliminarily measuring the frequency of acoustic signal giving a maximum received radio wave intensity, and emitting an acoustic pulse of this frequency.

The radio wave reflected by the acoustic face of an acoustic signal is received by a Doppler radar receiving antenna 7, guided to a radio wave receiver 8 to measure the radio wave received intensity by a received intensity measuring instrument 11. Since this radio wave received intensity is changed according to the frequency change of the acoustic signal, the maximum value of the received intensity is detected by a maximum received intensity detecting means 12. A frequency transmitter 1 selects the frequency of acoustic signal giving the maximum received intensity, and a measuring acoustic pulse of this frequency is formed by a pulse circuit 2, amplified by an acoustic amplifier 3, and emitted to the atmosphere from an acoustic device 4. According to this, the power can be saved by the shortening of the measuring time, and noise generation to the peripheral area can be significantly reduced.