There is disclosed a defect detecting apparatus that focuses a laser beam, irradiates it onto the surface of a sample to be examined, and detects a foreign substance/defect existing on the surface from the scattered light as a result of the irradiation of the beam onto the sample surface. In order to stably detect defects such as foreign substance, the defect detecting apparatus according to the invention is constructed to use a beam shape optical system by which the laser beam emitted from a laser source is shaped to change the illumination intensity from its Gauss distribution to a flat distribution so that the detected signal can be stably produced even if the relative position of a defect/foreign substance to the laser beam irradiation position is changed.