A defect inspection apparatus includes an irradiation optical system 20, a detection optical system 30, and an image processor 40. In the irradiation optical system, a mirror 2603 is disposed to reflect downward a beam flux that has been guided to a first or second optical path, and a cylindrical lens 251 and an inclined mirror 2604 are disposed to focus the beam flux that has been directed downward by the mirror, at an inclination angle from a required oblique direction extending horizontally, onto a substrate 1 to be inspected, as a slit-shaped beam 90.