To prevent damage of a dust-proof device for a hopper for powdered materials to be fed by flexible container bags and make replacement of the dust-proof device easier.

A dust-proof canvas shutter (10) is composed of an outer frame part (11), a rubber ring (14) provided in an opening (13) defined in a substantially central portion of the shutter (10), and a sheet (15) provided between the outer frame part (11) and the rubber ring (14) in a tensed state. A mounting groove part (3) opened upwardly is provided at an upper portion of a hopper (1) for powdered materials, and a gap A is provided in the mounting groove part (3) to insert the outer frame part (11) of the canvas shutter (10) thereinto. A canvas shutter holding member (4), which has a gap S between the holding member (4) and the upper end of the outer frame part (11) and which prevents the outer frame part (11) from being disengaged from the inside of the mounting groove part (3), is fastened above the mounting groove part (3).