There is provided an oral biofilm inhibitor having an exceptional inhibitory effect on oral biofilm formation. It is an oral biofilm inhibitor comprising a curable composition containing an antimicrobial agent, wherein a compressive strength of a cured product formed by curing the composition is 150 MPa or less, and a content of the antimicrobial agent is 0.001 to 3% by weight. An oral biofilm inhibitor thus obtained is used for inhibiting biofilm formation in an oral cavity by applying a curable composition containing an antimicrobial agent to a dental defect site for allowing the composition to cure at the dental defect site, and then disintegrating the cured composition.