A low solidity cascade diffuser (LSD) shows features of a wide operating range and a high pressure ratio in centrifugal compressors and blowers. According to the steady flow analysis results shown previously by the authors, the noise could be reduced effectively without deterioration of the LSD performance by means of a small blade tip-groove located at the shroud side, and a high blade loading could be achieved without stall even at the angle of attack as large as 15 deg. The high LSD performance at low flow rates was achieved by both formations of the stable and intense vortex in the shroud tipgroove and of the secondary flow moving circumferentially along the shroud wall toward the impeller exit. In the present study, the influence of the rotating jet-wake flow discharged from the impeller on the vortex in the shroud tip-groove and on the secondary flow moving circumferentially along the shroud wall were investigated by comparing the simulation results between the unsteady flow analysis and the steady flow one. It is clearly shown that the lift force of the LSD blade fluctuates periodically with the blade passing frequency, however, the flow behaviors of the vortex in the shroud tip-groove and the secondary flow on the shroud wall are little affected by the rotating jet-wake in the vaneless space.