The antihypertensive agent of the present invention contains a soluble silicon compound. The soluble silicon compound is preferably metasilicic acid or a salt thereof. In one embodiment, the antihypertensive agent of the present invention contains deep sea water containing the soluble silicon compound, or a dried product of such deep sea water. It is preferred that in such embodiment, the antihypertensive agent further contains metasilicic acid or a salt thereof. The amount of silicon present in the deep sea water in the form of silicic acid (SiO2—Si) is 25 μM or more. The deep sea water is typically collected at a depth of 300 m or deeper, and preferably at a depth of 600 m or deeper.