Provided is a solution suitable for oral administration of 7-[4-(4-benzo[b]thiophen-4-yl-piperazin-1-yl)butoxy]-1H-quinolin-2-one (compound (I)) or a salt thereof. A solution for oral administration containing compound (I) or a salt thereof, and at least one compound selected from the group consisting of lactic acid, phosphoric acid, glycolic acid, malic acid, tartaric acid, citric acid, succinic acid and acetic acid and having pH 2.5-4.5.
