PURPOSE: To make numerical expression of peaks caused by specific components possible by determining the starting and ending points of peaks caused by specific components appearing on a densitogram and then performing operations.

CONSTITUTION: Prominent values of data points in a densitogram I are determined and plotted on an X-Y coordinate thus obtaining a waveform II. A peak point P1 corresponding to a peak of M protein, for example, appears in the waveform II. Two points on the densitogram I corresponding to minimum values m1, m2 on the opposite sides of the point P1 are determined as the starting and ending points P, Q of a peak caused by M protein. The area S1 of a region defined by the X-axis and the densitogram between points Px, Qx on the X-axis corresponding to the points P, Q, and the area s2 defined by the X-axis and a line connecting the points P, Q between the points Px, Qx are then calculated. The difference ΔS between the areas S1, S2 corresponds to the area of peak caused only by the M protein. Fractional % of peak due to M protein is determined by dividing the differential area ΔS by the area of a region defined by the densitogram I and the X-axis between albumin fraction and γ-globulin fraction.