This paper describes the development status of the hypersonic precooled turbojet engine. A subscale engine "S-engine" is under development, which has 1.2 kN in thrust at sea-level static. The ground firing tests have been conducted to verify the engine performance and healthiness on the system and components. The Mach 2 flight test of the S-engine will be conducted in 2010. Fundamental researches are also conducted as follows. An unsteady simulation of liquid hydrogen fuel management helps to construct the engine start-up sequence. A system optimization of super/hypersonic vehicles using a multi-objective generic algorism shows that Mach 5 flight has superiority compared to Mach2 when the precooled turbojet engine is used as the propulsion system. A wind tunnel test of the rectangular inlet of the S-engine captured the Dailey type buzz.