The drift of a betatron tune from a RFKO driving tune reduces the betatron amplitude excited by RFKO. For the booster synchrotron in SPring-8, such tune drift occurs at on-demand operation for the top-up mode injection. To overcome this drift, we are developing a tune tracking RFKO system for bunch purification at the booster. In this system, The betatron motion of the main bunch was excited by positive feedback with which the excited amplitude is much less sensitive to the tune shift. The system is based on SPring-8 FPGA based bunch-by-bunch feedback processor and the kick signal produced by the feedback processor was send to the feedback kicker and the RFKO system for the purification. We describe the system and report some beam test result.