Abstract: A filter medium comprises nonwoven fabric of a lower fiber density and filter paper of a higher fiber density, the nonwoven fabric and the filter paper being superposed in a laminated structure. The nonwoven fabric and the filter paper are disposed upstream and downstream, respectively, in a direction in which a fluid to be filtered flows. Since the nonwoven fabric of a lower density is disposed upstream and the filter paper of a higher density is disposed downstream, relatively large particles are trapped by the upstream nonwoven fabric to promote the formation of a dust cake layer on the surface of the filter paper. Such surface trapping may prevent the particles from being embedded within the filter medium, thus preventing the filter medium from being clogged therein.