It is an object of the present invention to provide a method wherein a structure which was prepared with a biopolymer such as gelatin is chemically modified with the use of a low-volatile chemical substance without dissolution of gelatin, and a biopolymer crosslinking method for producing a biopolymer having high strength (a high degree of crosslinking). The present invention provides a method for producing a modified biopolymer, which comprises reacting a structure prepared with a biopolymer with a solid-state compound having a melting point of 50.degree. C. or more at a humidity of 50% or more, and a method for crosslinking biopolymer which comprises treating a biopolymer with a crosslinking agent, wherein the crosslinking agent concentration in a reaction mixture is 1.0% to 10% by weight and crosslinking is carried out in the presence of an organic fluorine compound.