The radial direction of the Peccei--Quinn field can drive cosmic inflation, given a non-minimal coupling to gravity. This scenario has been considered to be capable of explaining inflation, the strong $CP$ problem, and dark matter. We argue that Peccei--Quinn inflation is extremely sensitive to higher-dimensional operators. Further combining with the discussion on the axion quality required to solve the strong $CP$ problem, we examine the validity of this scenario. We also show that a resonant amplification of the axion field is unavoidable after Peccei--Quinn inflation.