PURPOSE: To measure the size of an association body in a dissociation/ association reaction of two molecules, by measuring elution volumes of respective molecule seeds varying the density of the one with a larger molecular weight to calculate a mass distribution coefficient and a boding coefficient of the association body from associated data on several densities and elution volumes.

CONSTITUTION: In this reaction measurement of a molecular association body, two molecule seeds different in the molecular weight are injected into a gel filtration column varying the density of the one with a larger molecular weight to measure elution volumes of respective molecule seeds. Relationship between density and elution volume is obtained in relation to characteristics such as mass distribution coefficient, bonding coefficient and the like pertaining to an association body in a dissociation/association reaction. This enables the calculation of the mass distribution coefficient and the bonding coefficient pertaining to the association body from associated data on density and elution volume.