A process of preparing rosiglitazone, or a pharmaceutically acceptable salt thereof, which process employs an intermediate metabisulphite complex of 4-[2-(N-methyl-N-(2-pyridyl)amino)ethoxy]benzaldehyde, which metabisulphite complex is represented by following formula (III): ##STR00001## where X represents an alkali metal. The present invention further provides rosiglitazone, or a pharmaceutically acceptable salt thereof, prepared by the above process.