To provide a clogging preventive apparatus which can always remove crushed stones, etc., to prevent the clogging of spaces between drain bars of a screening machine during feeding by a feeder by simple constitution.

A clogging preventive apparatus 100 arranged on the lower surface side of a screening machine 230 is composed of a large number of removing bars 10 which are arranged parallel to drain bars which constitute the screening machine 230 and some of which are inserted between them, a swing shaft 20 which integrates the removing bars 10, a swing arm which is installed in a part of the shaft 20, and a driving link 40 one end of which is connected with the swing arm and the other end of which is connected with the lower surface side of a receiving plate 220. By the reciprocation of the receiving plate 220, each removing bar 10 is brought into and out of each drain bar of the screening machine 230 to remove crushed stones, etc., which are going to cause the clogging of spaces between drain bars.