To provide a method and an apparatus for detecting dirt, which can securely detect the dirt of an ultrasonic sensor incorporated into a sludge interface meter, irrespective of the degree of dirt of drainage (sewage).

The variations with the lapse of time in the intensity of reflection waves near the ultrasonic sensor and near the bottom surface of the vessel to be measured are monitored, respectively. When the intensity of the reflection wave near the ultrasonic sensor increases and when the intensity of the reflection wave near the bottom surface of the vessel to be measured decreases, it is determined that the ultrasonic sensor becomes dirty.