The present invention relates to a contamination inhibitor containing at least one polyorganosiloxane that has silicon-bonded functional groups and that has a number average molecular weight of 3,000 to 500,000 in terms of standard polystyrene as determined by gel permeation chromatography and a silicon-bonded functional group equivalent (a number average molecular weight per mol of the silicon-bonded functional groups) of 3,000 to 500,000 on average. The contamination inhibitor of the present invention can effectively prevent or reduce adhesion of adhering substances on the surface of a substrate even under high temperature conditions, and exhibits superior stability, since the modified silicone having functional groups is used.