An intensive observation with ozonesondes, radiosondes, and the Middle and Upper atmosphere (MU) radar was conducted at Shigaraki (34.9¿N, 136.1¿E), Japan, from 16 through 24 April 1998 to investigate the formation mechanism of ozone laminae in the midlatitude lower stratosphere. In these observations the temporal variation of an ozone lamina at Shigaraki was continuously observed through a short time interval of 8 hours. During the observation period an ozone lamina with a thickness of 2--3 km appeared and ascended with a speed of ~1 km day-1 in the height region of 18--21 km. It is deduced that inertia gravity waves existing in this height region did not contribute to formation of the ozone lamina, because their amplitudes were too small and their vertical phase propagation was in the opposite direction. The reverse domain filling method was used to reconstruct the high-resolution modified potential vorticity field using backward trajectory calculations. The results indicate that this ozone lamina was caused by differential advection due to the vertical shear associated with the subtropical westerly jet and embedded stationary Rossby wave therein. This means that large-scale motions concurrent with vertical wind shear can produce much smaller-scale structures in material fields than those of the large-scale motions themselves.