The present invention provides a salt, particularly an ionic liquid, of a non-steroidal anti-inflammatory drug (NSAID) having carboxylic acid and an organic amine compound, which has the properties necessary for use as an external preparation (free of crystal precipitation, stable, easily soluble in organic solvents and the like often used for external preparations, and the like). The solving means is a 1:1 salt or ionic liquid of a carboxylic acid NSAID and an organic amine compound.