To make correctly detectable an edge by linearly combining a step luminance differential distribution and a spike luminance differential distribution with each other and detecting a corresponding edge only when the result matches with an object luminance distribution.

For example, on template Ts obtained from the spike luminance differential distribution and G templates Ti obtained from the step luminance differential distribution are prepared and a mask Mi is prepared by using Ts and Ti (S1). Then an object luminance differential distribution vector Y is processed with the mask Mi to obtain a vector Yi (S2). The similarity Qi between G templates (Ts-Ti) obtained by linearly combining Ts and Ti and the vector Yi is calculated, vectors which meet standard are checked (S3), and the vector having the highest similarity Qi among the meeting vectors is regarded as Yk (S5). Then, the spike and step luminance differential vectors rk and pk of Yk are calculated and when the meet standards, corresponding edges are found (S7).