Evolution and structure of multiple stall cells with short lengthscale in an axial compressor rotor have been investigated experimentally. In a low-speed research compressor rotor tested, a short lengthscale stall cell appeared at first, but did not grow rapidly in size unlike a so-called "spike-type stall inception" observed in many multi-stage compressors. Alternatively, the number of cells increased to a certain stable state (a mild stall state) under a fixed throttle condition. In the mild stall state the multiple stall cells, size of which was on the same order of the inception cell (a few blade spacings), were rotating at 72 % of rotor speed and at intervals of 4.8 blade spacings. With further throttling, a long length-scale wave appeared overlapping the multiple short length-scale waves, then developed to a deep stall state with a big cell. In order to capture the short length-scale cells in the mild stall state, a so-called 'double phase-locked averaging technique' has been developed, by which the flow field can be measured phase locked to both of the rotor and the stall cell rotation. Then, lime-dependent ensemble averages of the 3D velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and the pressure distributions on the casing wall with high response pressure transducers. By a physically plausible explanation for the experimental results, a model for the flow mechanism of the short length-scale stall cell has been presented. The distinctive feature of the stall cell structure is on the separation vortex bubble with a leg traveling ahead of the rotor, with changing the blade in turn on which the vortex leg stands.