Today turbocharging has become a fundamental technology to realize engine downsizing, which is an attractive strategy for low carbon emissions. High efficiency and wide operating range are strongly required for the automotive turbochargers. Especially centrifugal compressors for automotive turbochargers are requested to operate with high efficiency from the surge limit to the choke limit. The internal flow in a centrifugal compressor is however three dimensional and shows very complex unsteady flow phenomena like a rotating stall and a surging, which have yet to be elucidated fully. In this study the effect of flow path height of impeller and diffuser on flow fields in a transonic centrifugal compressor has been investigated both numerically and experimentally. Detached Eddy Simulation (DES) has been applied and revealed the reduction of impeller exit flow path affects the accumulation of low momentum flow at impeller inlet as well as impeller exit. Also it has been confirmed experimentally the 15% reduction of the impeller exit flow path height can halve the surging flow rate with same choking capacity at pressure ratio of 2.6.