PURPOSE: To accurately detect an abnormal migration image, by normalizing a specimen densitogram on the basis of the albumin peak positions and positions becoming equal to the predetermined threshold value of a γ-globulin peak skirt part in specimen and reference densitograms.

CONSTITUTION: In respective densitograms obtained by the electrophoresis of reference control serum and specimen plasma, albumin peak points Xci, Xsm and points Xcj, Xsn becoming equal to the predetermined threshold value Δy of a γ-globulin right skirt part are determined. Herein, since the size of a γ-globulin peak is greatly changed by the protein composition of the specimen plasma, said threshold value Δy is set according to the size of the γ-globulin peak so as to be capable of certainly determining the position Xsn. Next, the specimen densitogram 2 is increased by x-times and aligned in its length with the length of the reference densitogram to be normalized. Herein, the value of (x) is represented by x=(Xcj-Xci)/(Xsn-Xsm).