Recent research1 showed experimentally evidence of high-power and short duration microwave beam self-guiding in a plasma formed by ionization of gas at pressure of >100 Pa. In the present research we showed that the same self-guiding phenomenon occurs in the case of interaction between preliminary formed rf plasma with density in the range (0.8 – 5)10¹¹ cm^-3 and high-power (≤300 MW), sub-ns (~0.6 ns) microwave (9.6 GHz) beam. These experiments were carried at the at pressure of neutral gas (Ar, He or air) ≤10 Pa. Different diagnostic methods were applied to observe a self-guiding of the microwave beam along the axial distance of a few times longer than Rayleigh length. The low-dense neutral gas ionization process by plasma electrons with larger energy and smaller ionization rate close to the axis than its periphery, leads the microwave propagates through the guiding plasma channel. Analytic model and numerical PIC simulations confirmed the experimental result, describing the evolution of the plasma with preliminary non-uniform radial density profile.