Numerical simulations using RANS (SST-2003) and DDES (SST-2003sust) were conducted to capture the buzz phenomena which occurs on the air intake for High-Mach Integrated Control Experiment, HIMICO. HIMICO is a hypersonic flight experiment aiming at establishing the integrated control technologies of the airframe and the engine. The ramjet engine for HIMICO is composed of a variable air intake, a diffuser, a combustor, and a variable nozzle, and the size is Length:540 mm x Width:68 mm x Height:110 mm. A result of DDES simulation almost corresponds with the experimental result, but the result of RANS simulation estimates the incorrect frequency of the buzz. This error is caused by the existence of a separation region in the RANS simulation. This separation area makes the oblique shock wave from the ramp stronger, and the Mach number inside the intake smaller. In DDES simulation result, the shock wave structure downstream of the intake throat is adjusted to raise the pressure inside the engine. However, Mach number inside the engine is smaller in RANS simulation, so this adjustment is not carried out as well as DDES result, and the pressure increasing period becomes shorter in RANS result. Also, the shock wave structure obtained by the RANS simulation does not correspond with the experimental result while the shock is moving upstream. This is because the thicker separation region grows on the ramps while the shock wave is moving forward in the RANS simulation. Therefore, the type of the turbulence model greatly affects the existence and the shape of the separation region, which have great influence on the buzz characteristics, so the calculation methods must be selected carefully to simulate the buzz.