1. A pattern inspection apparatus comprising: a light source; a stage configured to mount thereon a substrate with a pattern formed thereon; a first laser measuring unit configured to measure a position of the stage by using a laser beam; a sensor configured to capture a pattern image obtained from the pattern, formed on the substrate, irradiated by light from the light source; an optical system configured to focus the light, which has penetrated the substrate on the stage, on the sensor; a second laser measuring unit configured to measure a position of the optical system, which focuses the light that has penetrated the substrate on the stage, by using a laser beam; a correction unit configured to correct a captured pattern image by using a difference between the position of the stage and the position of the optical system; and an inspection unit configured to inspect whether there is a defect of the pattern by using a corrected pattern image.