To provide a dust restraining device of a powder entrance slot capable of efficiently preventing dusts at a powder entrance slot and efficiently restraining dusts even when particularly a large amount of powder is put.

This dust restraining device 10 comprises an entrance slot 14 and a suction device 16 for sucking and discharging powdery dusts from powder put in a holding tank 24 to the outside of the holding tank, a circumferential vertical wall 18 provided at a position projecting to a center with a spacing from a side wall 22 of the holding tank while a top is closed, and forming a first suction path R1 communicated with the suction path of the suction device via a lower side of the circumferential vertical wall 18 from the entrance slot 14 when the input powder is smaller than a predetermined amount, and suction path securing means 20 for securing a second suction path R2 communicating the suction path of the suction device 16 with the outside of a holding layer via an opening 32 formed on the circumferential vertical wall 18 when the input powder becomes more than a predetermined amount.

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