Treatment agent for polyester synthetic fibers, composition containing treatment agent for polyester synthetic fibers, first treatment agent for polyester synthetic fibers, second treatment agent for polyester synthetic fibers, composition containing first treatment agent for polyester synthetic fibers, treatment method for polyester synthetic fibers, and polyester synthetic fibers

A first agent for treating a polyester synthetic fiber that is combined in use with a second agent for treating a polyester synthetic fiber containing a silicone (B) described below, the first agent for treating a polyester synthetic fiber comprising a silicone (A) described below, a nonionic surfactant, an anionic ingredient, and optionally a silicone (C) described below, wherein

• the amount of the silicone (C) contained in a mixture of the first agent for treating a polyester synthetic fiber and the second agent for treating a polyester synthetic fiber is less than 10% by mass,
• the silicone (A) is a silanol-modified silicone with a number average molecular weight of not less than 50,000 but less than 200,000,
• the silicone (B) is a silane coupling agent having at least one functional group selected from the group consisting of a methoxy group, an ethoxy group, an amino group, and an isocyanate group but not including an epoxy group in the molecule,
• the silicone (C) is at least one selected from the group consisting of silicone resins, dimethyl silicones, and alkyl-modified silicones but excluding those corresponding to being the silicone (A);
• wherein the nonionic surfactant includes at least one compound obtained by adding, in total, from 3 to 50 moles, inclusive, of an alkylene oxide to 1 mole of an alcohol, the alkylene oxide having from 2 to 3 carbon atoms, inclusive, and the alcohol having from 2 to 18 carbon atoms, inclusive, and having from 1 to 4 hydroxyl groups, inclusive.