Provided are a surface treatment agent for which hydrophobization to a high degree is possible even in a case of the material of a substrate surface being TiN or SiN, and surface treatment method using such a surface treatment agent. The surface treatment agent according to the present invention contains a cyclic silazane compound. As this cyclic silazane compound, a cyclic disilazane compound such as 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopentane and 2,2,6,6-tetramethyl-2,6-disila-1-azacyclohexane and a cyclic trisilazane compound such as 2,2,4,4,6,6-hexamethylcyclotrisilazane and 2,4,6-trimethyl-2,4,6-trivinylcyclotrisilazane are preferred. In the surface treatment, a substrate surface is exposed to a surface treatment agent according to the present invention, and the substrate surface is hydrophobized.