A photomask defect inspection method is provided by which defects of pin holes with the diameter equal to or less than 0.35 .mu.m can be detected with certainty. According to the inspection method, a pattern whose image is projected onto an imaging position by the use of illumination light (P1) for exposure consists of light transmitting portions (41) formed on a glass base (2) and light intercepting portions (42) which transmit part of the illumination light (P1) in such a way that a phase of the part of the illumination light (P1) passing through the light intercepting portions (42) is delayed with respect to a phase of the illumination light (P1) passing through the light transmitting portions (41). Slight detects in the photomask pattern are detected on the basis of a signal obtained by illuminating the pattern with inspection light having an inspection wavelength in which the transmittance (T) of the light intercepting portions (42) is defined in the following formula on the basis of a signal detection limit (Thr). When the signal detection limit (Thr) of an inspection circuit is calculated on the supposition that a signal level of the inspection light passing through the light transmitting portions (41) is equal to 1, the relational expression is T.gtoreq.(Thr-0.01).sup.1/1.8.