The present invention relates to a production method of a crystal of (R)-lansoprazole or (S)-lansoprazole, which includes crystallization at a temperature of about 0.degree. C. to about 35.degree. C. from a C.sub.1-4 alkyl acetate solution containing (R)-lansoprazole or (S)-lansoprazole at a concentration of about 0.1 g/mL to about 0.5 g/mL and the like. According to the production method of the present invention, a crystal of (R)-lansoprazole or (S)-lansoprazole superior in preservation stability can be produced efficiently on an industrial large scale.