PURPOSE: To improve resistance against noise by providing a received echo sound discriminating circuit which calculates propagation time from an SN ratio between a received echo sound level maximum value and a noise level value of a received waveform and a snow depth calculation circuit which calculates snow depth from frequency distribution of the propagation time.

CONSTITUTION: A transmission/reception circuit 2 is coupled to a echo sound transmitter/receiver 1, while the circuit 2 is coupled to a received echo sound discriminating circuit 5 via a received wave shaping device 3 and an A/D-converter 4. The circuit 5 is connected to a snow depth calculation circuit 6 and an auto gain control circuit (AGC) 9. The circuit 6 is coupled to a received wave following gate circuit 7 and an output circuit 8, while the circuit 7 is coupled to the circuit 5 respectively, and the circuit 9 is coupled to the circuit 2. At the time of measurement, the wave transmitter/receiver 1 receives transmission pulses of a constant interval to emit supersonic waves to a snow surface and also receives the reflected waves. The received pulses are subjected to AGC control by the circuit 9 and shaped by the shaping device 3. The circuit 5 calculates an SN ratio between the maximum value and a noise level value of a received wave sample value of received waveforms while waveforms with its ratio not reaching a received wave reference value are regarded as noise.