Provision of a stabilized crystal of 6-(3-chloro-2-fluorobenzyl)-1-[(S)-1-hydroxymethyl-2-methylpropyl]-7-meth- oxy-4-oxo-1, 4-dihydroquinoline-3-carboxylic acid (compound A). A crystal of compound A, which shows a particular X-ray powder diffraction pattern of a characteristic diffraction peaks at diffraction angles 2.theta.(.degree.) as measured by X-ray powder diffractmetry.