This paper proposes a constant altitude flight system using two dimensional laser range finder with mirrors. Recently, unmanned aerial vehicles (UAVs) are expected to perform in the inspection of deteriorated infrastructures such as bridges and tunnels. In the most of such operations, UAVs should be navigated and controlled in GPS-denied environment. In this paper, we apply two dimensional laser range finder with mirrors that can scan over horizontal direction and can measure vertical direction by using the change of the laser direction with mirrors. Consequently, the sensor makes it possible to achieve three dimensional environmental recognition. This paper proposes altitude estimation method from the point data obtained by the sensor and demonstrates constant altitude flight experiment using that two dimensional laser range finder with mirrors. The flight experiment verified the validity of the developed system.