1. A method for detecting defects on an object, comprising the steps of: obliquely illuminating the object with slit shaped laser light, the slit shaped laser light being converged in one direction and collimated in a direction transverse to the one direction; detecting with a first detection optical unit a first image formed by light reflected from the object by the illumination of the slit shaped laser light and reflected in a first oblique direction to the surface of the object; detecting with a second detection optical unit a second image formed by light reflected from the object by the illumination of the slit shaped laser light and reflected in a second oblique direction to the surface of the object; processing with an image signal processing unit both of a signal outputted from the first detection optical unit by the detection of the first image and a signal outputted from the second detection optical unit by the detection of the second image; and outputting information processed by the image signal processing unit; wherein the step of detecting with the first detection optical unit includes cutting off light reflected from patterns formed on the object by utilizing a first variable spatial filter; and wherein the step of detecting with the second detection optical unit includes cutting off light reflected from patterns formed on the object by utilizing a second variable spatial filter.