A compound which can be used as an agent for treating interstitial cystitis. An agent for treating interstitial cystitis, which comprises a phenoxyacetic acid derivative represented by a general formula (I) ##STR00001## wherein R.sup.1 represents hydroxyl group or a lower alkoxy group, or a pharmacologically acceptable salt thereof. Since the phenoxyacetic acid derivative represented by the general formula (I) or a pharmacologically acceptable salt thereof or a hydrate or solvate thereof has a selective .beta..sub.3-adrenoceptor stimulating activity and a remarkable action to inhibit a capsaicin-sensitive sensory nerve, it can be used as a preventive or therapeutic agent for interstitial cystitis and hyperactive bladder which are accompanied by pain, other diseases in which the capsaicin-sensitive sensory nerve is concerned (e.g., acute or chronic and systemic or topical pain and inflammation) and irritable bowel syndrome (IBS) and the like.