The present invention provides 3-{(2S,4S)-4-[4-(3-methyl-1-phenyl-1H-pyrazol-5-yl)piperazin-1-yl]pyrrolidin-2-ylcarbonyl}thiazolidine (compound I) useful as a dipeptidyl peptidase-IV inhibitor, which has superior properties of stability and hygroscopicity, and reproducible crystal structure, and a production method thereof.