In this study, bypass door area for the Hypersonic Precooled Turbojet Engine to restart the variable intake is estimated. Total pressure recovery and mass capture ratio of the variable air intake is acquired by the supersonic wind tunnel testing of the half scale intake model. Pressure loss and temperature effectiveness of the precooler is acquired by direct connect firing tests of the engine. Using these results, area of the bypass door by which the intake can start is estimated. The bypass door area to restart the intake depends on the precooler’s cooling capability. If the engine runs on liquid hydrogen, area of the bypass door is 1800mm2~2000mm2(Abyp_eng/A0=0.26~0.29). If the liquid nitrogen is used for the coolant of the precooler, area of the bypass door is 2600mm2~2700mm2(Abyp_eng/A0=0.37~0.39).