To provide a scum skimmer capable of reducing the amount of sucked water irrespective of variation in water level, and accordingly, always efficiently sucking scum.

This scum skimmer is provided with a weir means which is upwardly and downwardly movable with the water surface in a treatment pond such as rectangular or circular sedimentation pond, as the reference plane, a scum delivery member for receiving scum introduced by downward movement of the weir means and delivering the scum to the outside, a weir drive means for upwardly and downwardly moving the weir means; and a sensor means placed between a member upwardly and downwardly moving corresponding to the variation in water level and a moving member of the weir drive means, and by these sensor means, the weir can be controlled so as to be lowered always to a specified depth from the water surface and stopped there, correspondingly to the vertical variation in water level.