A precooled turbojet engine has been developed by JAXA for the hypersonic airplane and spaceplane. The subscale engine named "S-engine" whose thrust and weight is about 1.2 kN and 100 kg was designed and fabricated. Components and the system firing tests under the sea-level-static condition are successfully conducted. As the next phase, a flight test of the S-engine is planed using a stratospheric balloon in 2009 called "BOV: Balloon based Operation Vehicle". The vehicle is dropped from an altitude of 40 km by a high altitude balloon. After 40-second free-fall, the vehicle is pulled up and the S-engine operates for 30 seconds at about Mach 2. Corresponding to the BOV flight trajectory, high-altitude tests of the core-engine are conducted.