A filtration medium for a filter, having high dust collection efficiency, low pressure loss, a long service life and sufficient processing strength into a filter. A combined fiber nonwoven fabric includes first fibers having a mean fiber diameter of less than 200 nanometers, and second fibers having a mean fiber diameter in the range of 200 to 5000 nanometers, in which basis weight of the combined fiber nonwoven fabric is in the range of 2.1 to 15.0 g/m2.