Provided is a monomethanesulfonate of a phenylpyrrole derivative, having superior glucokinase activating activity and demonstrating remarkably improved solubility, hygroscopicity and stability as well as superior oral absorption, and crystals thereof. The present invention provides (2S)-2-(3-{5-[(5S)-5-methyl-4,5-dihydro-1,3-oxazol-2-yl]-1H-pyrrol-2-yl}-5-{[5-(methylsulfonyl)pyrazin-2-yl]oxy}phenoxy)propan-1-ol monomethanesulfonate having superior glucokinase activating activity, crystals thereof, a pharmaceutical containing the same, and a preventive and/or therapeutic agent for diabetes and the like.