PURPOSE: To enhance the bleaching action of a microbial cell on pulp by treating residual lignin of an unbleached kraft pulp with ozone and then decomposing the lignin with white-rot fungi.

CONSTITUTION: The method for bleaching pulp with ozone is to treat residual lignin of unbleached kraft pulp with ozone, then oxidize the aromatic nucleus thereof and subsequently react white-rot fungi such as Phanerochaete.chrysosporium therewith, decompose the lignin and provide the bleached pulp. Thereby, the treating time can be shortened.