PURPOSE: To make it possible to measure altitudes regardless of the change in attitude such as in diving of aircraft.

CONSTITUTION: A plurality of antennas 111-116 are attached around a fuselage 12 of an aircraft so that the pointing direction of the antenna becomes the radiating direction with the axial center of the fuselage as the center. These antennas are controlled by a switching control part 18 and sequentially connected to a transceiver 16 one after another. This operation is repeated. Under the state wherein one antenna is connected to the transceiver 16, the pulse radio wave is radiated from the transceiver 16 under the control from a pulse generating circuit 17. The reflected wave is received, and the altitude is measured. At this time, at least one antenna is connected to the transceiver during the time corresponding to the measurable longest distance. In this way, the altitude measurement with each antenna is performed. The shortest distance, i.e., the low altitude, is outputted as the true altitude with an altitude detecting part 21 at every time the altitude is measured with respect to all antenna.