A filtering device comprising a primary filtration unit for filtering a raw water, which unit includes a filtering medium and a housing for accommodating the filtering medium: in which the housing has defined therein a raw water supplying port for supplying the raw water therethrough into the filtering medium, a filtrate water outlet port, a fluid supplying port for supplying a fluid for backwashing therethrough into the filtering medium and a drain port for discharging therethrough the fluid used to backwash the filtering medium and the raw water; in which the filtering medium includes a hollow cylindrical depth filter of an external pressure type having a closed end and an open end oriented towards the filtrate water outlet port to thereby communicate a hollow of the depth filter with the filtrate water outlet port, said depth filter being prepared by a web of synthetic fibers, a non-woven fabric, paper or textile fabric welded and/or molded and having a pore size within the range of 1 to 25 μm; and characterized in that the filtering device further comprises a filtrate supplying passage fluid connected with the filtrate water outlet port, a drain passage fluid connected with the drain port, a communication passage for communicating the filtrate supplying passage with the drain passage, a second switching valve for selectively opening or closing the filtrate water supplying passage downstream of the connection with the communication passage, a fourth switching valve for selectively opening and closing the drain passage between the primary filtration unit and the communication passage, and a fifth switching valve for selectively opening or closing the communication passage. in which the housing has defined therein a raw water supplying port for supplying the raw water therethrough into the filtering medium, a filtrate water outlet port, a fluid supplying port for supplying a fluid for backwashing therethrough into the filtering medium and a drain port for discharging therethrough the fluid used to backwash the filtering medium and the raw water; in which the filtering medium includes a hollow cylindrical depth filter of an external pressure type having a closed end and an open end oriented towards the filtrate water outlet port to thereby communicate a hollow of the depth filter with the filtrate water outlet port, said depth filter being prepared by a web of synthetic fibers, a non-woven fabric, paper or textile fabric welded and/or molded and having a pore size within the range of 1 to 25 μm; and characterized in that the filtering device further comprises a filtrate supplying passage fluid connected with the filtrate water outlet port, a drain passage fluid connected with the drain port, a communication passage for communicating the filtrate supplying passage with the drain passage, a second switching valve for selectively opening or closing the filtrate water supplying passage downstream of the connection with the communication passage, a fourth switching valve for selectively opening and closing the drain passage between the primary filtration unit and the communication passage, and a fifth switching valve for selectively opening or closing the communication passage.