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 (SiO.sub.2--Si) is 25 .mu.M 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.