A gas generator using the catalytic decomposition of ethanol-blended hydrogen peroxide was investigated for application in experimental research on high-speed airbreathing engines. The gas generator should supply product gas with an oxygen concentration of 21 mol % at a given pressure of 3–12 bar and a temperature near 500°C, which are the flow conditions in a ramjet combustor flying at Mach 3. To avoid detonation of the liquid monopropellant, an appropriate mixture ratio of hydrogen peroxide, water, and ethanol was selected. In the batch reactor test, the tested catalysts were manganese oxide and platinum with gamma-alumina pellets. The temperature measurements indicated that the platinum with alumina pellets was more appropriate for decomposing propellant than manganese oxide with alumina pellets. In addition, product gas species with an oxygen concentration of 21 mol % were confirmed through the test. In the demonstration test, the system was operated with a propellant mass flow rate near 100 g/s. A catalytic reaction was sustained for the whole operation time with an adiabatic decomposition temperature efficiency of over 0.98. The measured flow temperature was near 500°C and the chamber pressure was near 5.5 bar, which satisfied the flow conditions in a ramjet combustor flying at Mach 3.