A main object is to provide a crystal of 6-fluoro-1-methyl-7-[4-(5-methyl-2-oxo-1,3-dioxolen-4-yl)-methyl-1-piperazinyl]-4-oxo-4H-[1,3]thiazeto [3,2-a]quinoline-3-carboxylic acid acetonitrile solvate (Compound B) which is anintermediate for producing preferentially the type III crystalof 6-fluoro-1-methyl-7-[4-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl-1-piperazinyl]-4-oxo-4H-[1,3]-thiazeto [3,2-a]quinoline-3-carboxylic acid (Compound A).A crystal of Compound B can be preferentiallyprecipitated by controlling the supersaturation concentrationin crystalization using acetonitrile as a solvent.Subsequently, the type III crystal of Compound A can be producedby performing desolvation of the crystal.
