To provide a dust flocculating passage of simple structure capable of promoting the flocculation of particles in dust by increasing the number of particle collisions, thereby to reduce the particle number and to enlarge the apparent particle diameter.

The dust flocculating passage 90 includes a passage 91 for a gas containing the dust to pass therethrough, and a frictional electrification route for electrifying the dust flowing in the passage 91 and promoting the generation of dust lump (a cluster) by allowing the electrified dust to collide with the other dust. The frictional electrification route includes a wall 92 for forming the frictional electrification route. Projections 93 having a projecting corner part are arranged on the inner surface of the wall 92. The corner part of each projection 93 has a round part 94 with the end surface rounded therein, where the radius of the round part 94 is not less than 0.2 mm.