A treating agent for filling synthetic fibers, said treating agent comprising (a) 1 part by weight of an aminosilane of the formula (I) ##EQU5## wherein R1 is a hydrogen atom, an alkyl group containing 1 to 4 carbon atoms, or a phenyl group, R2 is an alkylene group containing 1 to 4 carbon atoms, and R3, R4, and R5, independently from each other, represent an alkyl group containing 1 to 4 carbon a