The human visual system is a sensory organ having a very high performance. It analyzes various visual information at a very high speed. The key of such performance will be the parallel structure of the visual system. As the fundamental process of the human visual system, the authors pay attention to the achromatic information processing system. In order to elucidate mechanism of the achromatic channel, in the present paper, the achromatic perception has been measured by three psychological experiments. In the first experiment, the spatial characteristic of the achromatic perception has been measured for six wavelengths of monochromatic light. In the second experiment, the relative spectral sensitivity of the achromatic perception has been measured for two test sizes. In the third experiment, the relative spectral sensitivity of the achromatic perception has been measured for two background levels, i.e., at 55 and 798 Td. It is difficult to explain the experimental results by the conventional achromatic channel model, and the experimental results suggest that the achromatic information is processed by several subsystems in parallel. In order to explain the experimental results, we propose a model of achromatic information processing composed of five stages and three subsystems.