The time dependency of pressure drop across a flat type fabric filter coupon was experimentally studied by releasing the accumulated dust cake on the filter. It was found that increasing pattern of pressure drop could be divided into three stages in each filtration cycle. In the first stage, the rate of increase in pressure drop rises; in second stage, it decreases; and in the third stage, it is almost a constant. In the first and second stage, the slope of pressure drop curves strongly depends upon the filtration cycle and the pressure of cleaning air. In a given filtration cycle, the higher the tank pressure is, the pressure drop increases the more slowly. At given tank pressure, the rate of increase in pressure drop increases with the number of filtration cycle. In the initial several filtration cycles, the pressure drop increased concavely at higher tank pressure. The concave increase was considered to occur because the cleaning force applied to the fabric was too strong, so that dust that had penetrated the fabric was removed. If too much dust is removed from inside the fabric, this may delay the formation of so-call primary dust layer.