To analyze the phenomenon of increasing pressure drop during dust removal using a ceramic filter system under high temperature conditions, the pressure drop in an ash powder layer on a ceramic filter during the gas permeation and dust detachment processes was directly measured with a small-scale testing apparatus. Dust detachment behavior of the ash layer on the ceramic filter was evaluated by using both quasi-static and pulse reverse flow methods. Under conditions of relatively low temperatures (below 1050 K), the pressure drop in the gas permeation test was almost fixed, and the maximum pressure drop (Deltap(max)) at the first detachment of a dust layer by quasi-static reverse flow increased gradually in proportion to temperature. At temperatures higher than 1123 K, a time-dependent increase of pressure difference during gas permeation occur-red before dust detachment and a rapid increase of Deltap(max) during dust detachment was observed. The time-dependent densification of ash layers and a rapid increase of adhesion between ash particles were observed by thermo-mechanical analysis (TMA) and a diametral compression test of ash powder pellets. In this paper, we suggest that the pressure drop increase is controlled by a decrease of pore volume and an increase of fracture strength within the ash layer.