A device for compacting a moulding mixture in which the latter is subjected to the action of compressed air by supplying the air into a reservoir under a pressure exceeding 20 kg/cm2 in such an amount that 3 to 40 volumes of air under a normal pressure are provided per unit or volume of the mixture being densified. Then the reservoir is discharged during a time interval not exceeding 0.15 sec. by directing the flow of air into the mixture in a moulding box. The reservoir is formed by a chamber of a housing and at the external side of this housing under a discharge opening closed by a valve there is mounted a plate forming an additional chamber. The plate has holes for a uniform distribution of the air over the surface of the mixture being compacted.