A plate inspection system and a plate inspection method with which irregularities in phase difference caused in a retardation layer can be efficiently detected. The inspection system is for inspecting a plate to be inspected having a retardation layer. The plate inspection system comprises a polarized-light source for irradiating a polarized light and an observation-side polarizer placed on the observation side. In the inspection system, a plate to be inspected is placed between the polarized-light source and the observation-side polarizer so that the plate to be inspected is irradiated with polarized light from the polarized-light source. The position of at least the observation-side polarizer or the plate to be inspected is changeable relative to the polarized-light source.