1. A pattern defect inspection apparatus for detecting defects by comparing a detection image obtained by an image sensor, patterns having an identical shape and being continuously laid out on an object to be tested at equal intervals in row and column directions, with a reference image obtained by scanning patterns of the identical shape neighboring in the row and column directions thereof, said apparatus comprising: a statistical computation unit which calculates a quadratic curved plane which is adaptable to two-dimensional distribution of signal intensity on each of image pixels of identical shape patterns lying next to the detection image including a predetermined number of nearest chips on up-and-down and right-and-left sides with the detection image being intermediately situated according to the signal intensity on image pixels; and a unit which detects a defect by comparing a signal intensity of an image pixel at first a (x,y)-coordinate of the detection image with a signal intensity at second a (x,y)-coordinate of said quadratic curved plane corresponding to said first (x,y)-coordinate.