PURPOSE: To obtain a differential pressure type altimeter of high precision which can be used practically for photogrammetry by measuring a differential pressure between a closed space of which the temperature is kept prescribed and the outside air.

CONSTITUTION: A semiconductor differential pressure sensor 1 having a strain gage structure is fitted to a case body 20 forming a close space 2 and a construction is made so that a pressure difference between the closed space 2 and the outside air is shown as a voltage in the sensor 1. First a temperature regulating means 10 comprising a temperature sensor 4, a temperature control device 6, a heater 3 and a power source 5 is operated so as to keep the closed space at a prescribed temperature, and the scale is set at zero (m). When an unmanned small-sized helicopter is made to rise in this state and an external atmospheric pressure lowers, an atmospheric pressure difference occurs between the external atmospheric pressure and an internal atmospheric pressure of the closed space 2 and corresponding to the differential pressure is indicated. When this altitude reaches a prescribed value, the helicopter is made to conduct hovering and a prescribed area is photographed. The precision of this differential pressure type altimeter is about 0.5m (0.5g/cm2), and even when photographing is conducted at an altitude of 100m, a difference from the real altitude can be processed within the range of an error.