[Problem] To provide a contamination preventing agent composition capable of effectively preventing pitch contamination in a dry part. [Solution] The present invention relates to a contamination preventing agent composition for preventing pitch contamination in a dry part D of a papermaking process, the composition containing: a linear polysiloxane compound represented by formula (1); and a cyclic siloxane compound. [In formula (1), a substituent R1 represents, in the same molecule, a hydrogen atom, an alkyl group, a methylphenyl group, a polyether group, a higher fatty acid ester group, an amino-modified group, an epoxy-modified group, a carboxylic group, a phenol group, a mercapto group, a carbinol group, or a methacrylic group, and a repeating number n of a siloxane unit represents an integer of 20-1430.]