A device for suppressing slit clogging capable of maintaining recovery rates of ground particles without clogging of the ground particles in an opening is provided. The device for suppressing slit clogging is used in a rotating-drum type magnetic separator 1 which includes a rotating drum 3 on which a plurality of magnets 4 are disposed and separates unnecessary matters in treated coolant liquid. A slit-like opening 8 is provided below the rotating drum 3, and the device includes a cylindrical groove cam mechanism, which is configured by a cylindrical shaft 21 having a groove which can be rotated and a slit cleaning member 22 which engages with the groove, below the opening 8. The slit cleaning member 22 moves along a longitudinal direction of the opening 8.