A cocrystal of (1S)-1,5-anhydro-1-[3-(1-benzothien-2-ylmethyl)-4-fluorophenyl]-D-glucito- l and L-proline. It is a cocrystal of known compound A, which has a constant quality, is superior in storage stability, has no moisture absorptivity, and is suitable as a crystal of a drug substance used for preparing pharmaceuticals.