Provided is a cyclic siloxane represented by general formula (1) (wherein R1, R2, R3, X, a, b, c, R4, Y, and d are as defined in the description), the cyclic siloxane having a hydrolyzable functionalgroup that comprises an alkoxysilyl group and having a linear polysiloxane structure having a specific length. The compound is utilizable as a surface treatment agent which is for thermally conductivefillers and is capable of giving compositions excellent in terms of applicability, thermal conductivity, and operating efficiency.