To provide the distance measuring device for both short distances and long distances, which definitely performs the judgement as to which of the measured results of the distance measurement of a FM-CW method of a pulse method has the acuurate value.

The reflected wave of the transmitted wave from an airframe from the ground surface is received by a receiving part 5 and outputted to a frequency mixing part 3 as the reflected wave IW1. The frequency mixing part 3 obtains a beat signal BT based on the frequency difference between the reflected signal IW1 and a transmitted signal OW1 transmitted from a transmitting part 1 and outputs the signal to a beat-signal detecting part 7. Then, the beat-signal detecting part 7 performs the detection of the beat signal and outputs the detected signal to a controller 8. Then, the controller 8 selects the method for the distance measurement by the FM-CW method having the short measuring range when the beat signal is detected. Furthermore, the controller 8 selects the method for the distance measurement by the pulse method having the long frequency range when the beat is not detected by the beat-signal detecting part 7.