Salt of indole derivatives and crystal thereof

The present invention addresses the problem of providing a different mode of a 4-(3-cyanoindol-1-yl)-2-hydroxybenzoic acid which has xanthine oxidase inhibitory activity and is useful as a preventive and therapeutic agent for diseases caused by abnormalities in the serum uric acid level. The present invention provides: a 4-(3-cyanoindol-1-yl)-2-hydroxybenzoic acid sodium salt which has excellent solubility and other physical properties, is useful as a drug substance, and is suitable for industrial production of a medicine; and a method for producing the same.