A treatment device such as a biliary stent, capable of effectively and continuously inhibiting biofilm formation, and a biofilm formation inhibitor suitable for coating on the treatment device by killing all bacteria including E. coli and by suppressing the growth of these bacteria, are provided. A biofilm formation inhibitor having an effective amount of catechin for inhibiting biofilm formation and a carrier that is suitable for adhesion of said catechin to a treatment device to be placed in the living body, are provided.