The invention relates to purification of (2S,3S)-3-[[(1S)-1-isobutoxymethyl-3-methylbutyl]carbamoyl]oxirane-2-carboxylic acid [hereinafter, referred to as the carboxylic acid] using the salt of the carboxylic acid with an organic amine selected from the group consisting of piperazine, adamantane amines and others. The invention also relates to providing crystalline sodium salt of the carboxylic acid. The sodium salt is usable as a material for preparing medicaments and improved in storage stability, and has the following characteristics: DSC: exothermic peak observed at a temperature in the range of 170 to 175° C. with weight decrease; and characteristic absorption bands of infrared absorption spectrum (KBr tablet) 3255, 2950, 2860, 1670, 1630, 1550, 1460, 1435, 1395, 1365, 1310, 1260, 1110, 890 cm−1.