A type IV collagen high molecular form having a higher molecular weight than the 7S domain of type IV collagen and including the 7S domain in its structure, is obtained from the supernatant being recovered from a collagen solution digested by pepsin in the following steps;1) salt precipitating with sodium chloride at a concentration no higher than 1.2 M,2) dissolving the precipitates,3) salt precipitating with sodium chloride at a concentration no higher than previous concentration. By reacting a sample with an antibody which reacts with this form, the type IV collagen high molecular form in the sample can be measured to allow diagnosis of the degree of liver fibrosis in patients with liver diseases.