© 2019 Japan Association for Wind Engineering. All rights reserved. The flow behind two flat plates in a parallel anangement was experimentally investigated by varying the gap ratio (G/D) of the gap spacing (G) between the plates to the plate width (D). The flow patterns around the plates were observed using the hydrogen bubble flow visualization. Velocity components were acquired by employing hot-wire anemometers in the wind tunnel. From the investigation, it has been found that, at gap ratios of 0.1 ≤ G/D < 1.6, the gap flow was biased either upward or downward in a stable way. At 1.6 ≤ G/D < 2.0, the switching of the biased gap flow was observed due to the vortex interaction behind the plates, while at G/D ≥ 2.0, the gap flow was no longer biased. At gap ratios less than 2.0, two Strouhal numbers indicating the bi-stable state exist, and the vortex shedding frequency from the plate on the biased side decreases with the gap ratio.