PURPOSE: To simplify circuit constitution by specifying the frequency of a radio wave to be inputted to a receiver, and finding an altitude signal from the shift in transmission frequency of a transmitter.

CONSTITUTION: An amplified altitude frequency outputted from an amplifier 7 is discriminated by a frequency discriminator 13, whose output is inputted to a voltage-controlled oscillator 2. When said output is received to vary an oscillation frequency so that the output signal of the frequency discriminator 13 has a zero level, the frequency of a radio wave transmitted from a transmitting antenna A1 and that of a radio wave received by a receiving antenna A2 are both specified. The frequency shift FD of the voltage-controlled oscillator 2 is detected and inputted to an altitude voltage generator 8 to control the charging and discharging time of a capacitor, and thus a voltage corresponding to an altitude is generated to display the altitude on an indicator 9.