A dust collecting mechanism for an air pump, in particular, an electromagnetic vibrational pump having a compact structure, capable of realizing the efficiency of dust collecting ability is disclosed. The mechanism is simple in assembly work. An inlet and an outlet for fluid are provided in the mechanism at a barrier chamber of the mechanism provided in at least one portion of a housing. A plurality of cross walls at which 1 or 2 or more of vent holes are provided are arranged in the mechanism. At least the position of the vent hole of the adjacent cross wall is different among a plurality of the cross walls.