PURPOSE: To analyze the free part of an organic matter which exists in a biological fluid in a state where the matter is bound with a protein and another state where the matter is free from the protein by causing a reaction among a free ligand, its labeled derivative, and a specific binder.

CONSTITUTION: A biological fluid sample is mixed with the labeled derivative of a ligand and a specific binder for the ligand so as to cause a reaction and the amount of the labeled derivative of the ligand bound or not bound with the binder is measured. The measured value is used for the measurement of the concentration of the free ligand in the biological fluid. In this case, the labeled derivative of the ligand is adjusted so that the derivative can be bound with the specific binder, but cannot or very weakly bound with a natural binder, and the used amount of the specific binder is adjusted to a sufficiently small value so that the balance between a ligand binding type binder and a free binder cannot be lost.