An improved homogeneous enzyme immunoassay process for quantitatively analyzing an antigen by determining the change in the enzymatic activity caused by a reaction between the antigen and an enzyme-labeled antibody. The antigen is reacted with an enzyme-labeled antibody, followed by the reaction with a second antibody capable of recognizing and binding to a different epitope and then with a third antibody capable of recognizing and binding to the second antibody. The enzymatic activity of the labeling enzyme is determined by a water-insoluble substrate. Using the water-insoluble substrate, steric hindrance is enhanced. A highly-sensitive analysis can be carried out by a simple operation even when the antigen has a molecular weight falling within an intermediate range, for example, a range of M.W. 10,000 to 70,000.