A polarized proton beam was accelerated at the KEK‐PS in the late 1980’s. This report will review the activities for the polarized beam acceleration in both the 500 MeV booster and the 12 GeV Main Ring. Some depolarization resonances were passed using the spin‐flip techniques. The effects on the polarization caused by a synchrotron side‐band at a strong intrinsic resonance were observed. © 2003 American Institute of Physics