To provide a scum detection method and a scum detection apparatus for detecting scum by a simple method and for removing the scum.

In the scum detection method, a water level in a sludge precipitation layer 10 and the flow rate in a treated water channel 32 are measured by an ultrasonic sensor 34, the generation of scum is detected by the difference in the water level and flow rate that has been calculated, and a scum-scratching machine 20 is operated. Therefore, the scum detection apparatus 2 has a water level measurement means in the sludge precipitation bath and a low rate measurement means in an overflow channel, and has an operation section 40 and a control section 42, and automatically operates the scum-scratching machine 20 by the control section 42.