To provide a method for treating lignin or a vegetable fiber comprising the lignin by repeatedly using manganese peroxidase, efficiently affording trivalent manganese in stable yield, and using the resultant trivalent manganese, and to provide the vegetable fiber obtained by the method for treating.

The method for treating the lignin or the vegetable fiber comprising the lignin in the presence of the trivalent manganese is provided. In the method, the method for producing the trivalent manganese has: a first step of reacting the manganese peroxidase with divalent manganese and an oxidizing agent in water or an aqueous solution or a mixed solvent of the water or the aqueous solution with an organic solvent, oxidizing the divalent manganese, and thereby producing the trivalent manganese; and a second step of separating the manganese peroxidase from a reaction solution containing the trivalent manganese by fractionation using a membrane. Furthermore, the method for treating the lignin or the vegetable fiber comprising the lignin is provided.