The experimental study investigates the cause of prestall, which the magnitude increases of the shaft frequency near stall. This prestall phenomenon is related to the geometric non-uniformity of the axial compressor, which can be classified into blade non-uniformity and casing non-uniformity. The instant static pressure was decomposed into several signal components to investigate this phenomenon. To verify the blade non-uniformity, the dimensionless revolution aperiodic component (${psi}$) distribution, which was measured at one arbitrary circumferential location, was analyzed, and to verify the casing non-uniformity, the amplitude of ${psi}$ was analyzed at 8 equally spaced circumferential locations of the first stage. The measurements showed that the blade non-uniformity directly caused the increase of the magnitude of the shaft frequency near stall but that the casing non-uniformity induced the increase of the magnitude of the shaft frequency in the Seoul National University compressor.