A defect inspection method comprises irradiating a sample including a pattern under inspection with light, acquiring measurement pattern data of the pattern based on intensity of light reflected by the sample, generating conversion data including pixel data corresponding to the measurement pattern data from design data of the sample, applying FIR filter process to the conversion data, reconstructing the conversion data by replacing pixel data having value not larger than first reference value with first pixel data, replacing pixel data having value larger than second reference value larger than first reference value with second pixel data having value larger than first pixel data, replacing pixel data having value larger than first reference value and less than second reference value with third pixel data having value between the value of first and second pixel data, the pixel data having larger value being replaced with third pixel data having higher value.