An optical inspecting apparatus includes a first light source, a beam splitter, a first lens, a first light detector, and pinhole plates. The first light source emits a first light beam. The beam splitter transmits or reflects the first light beam. The first lens provides the first light beam to transmit through a transparent substrate of a photomask and forms a first focusing spot on a first surface of the transparent substrate or a top surface of a photomask pattern formed on the transparent substrate. The first light detector detects a first reflection light beam generated by reflecting the first light beam from the first surface of the transparent substrate or the top surface of the photomask pattern. The pinhole plates are disposed in front of the first light detector to filter noise in the reflection light beam.