Disclosed is a technique for improving the water solubility and storage stability of 7-{(3S,4S)-3-[(cyclopropylamino)methyl]-4-fluoropyrrolidine-1-yl}-6-fluoro-1-(2-fluoroethyl)-8-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (compound (1)) that is safe and not only has a strong antibacterial action but also is effective for resistant bacteria for which conventional antibacterial agent are less effective. Crystals of the hydrochloride salt of the compound (1), crystals of the hydrochloride salt hydrate of the compound (1), and crystals of the methanesulfonate salt of the compound (1) are provided. In these crystals, decomposition due to influences of light is suppressed as compared to that in crystals of the compound (1) in free, and their storage stability is high. These crystals have higher solubility in water than the crystals of the compound (1) in free.