1. A defect inspection method for acquiring image signals from a substrate that is to be inspected, the substrate having circuit patterns repeatedly formed thereon, and inspecting a defect present on the substrate on the basis of the acquired image signals, the method comprising the steps of: setting a target local region and a plurality of corresponding local regions in the acquired image signals, the target local region including a target pixel and an area surrounding the target pixel, the corresponding local regions including pixels corresponding to the target pixel and areas surrounding the corresponding pixels; forming a joint histogram feature space of the corresponding local regions on the basis of the image signals of the corresponding local regions set in the local region setting step; determining a threshold on the basis of the joint histogram feature space formed in the forming step; and detecting the defect present on the target local region on the basis of differential images between the target local region and the corresponding local regions using the threshold determined in the threshold determining step.