To provide a fouling detection method of a sensor for determining existence of fouling adhering to the sensor, especially an optical sensor, and a cleaning method of the sensor for cleaning the fouling adhering to the sensor.

In a sensor 1 for determining the suspended matter concentration in water M to be measured, or in a sensor 1 for detecting physical properties of the suspended matter, existence of fouling of the sensor 1 is determined by comparing the difference between a measured value determined by measuring the water M to be measured beforehand in the state where the sensor 1 is not fouled and a measured value determined by measuring the water M to be measured after introducing bubbles between a floodlighting part 2 and a light receiving part 3 in this state, with the difference between a measured value determined in a measuring process for measuring the water M to be measured by using the sensor 1 and a measured value determined when the bubbles are introduced between the floodlighting part 2 and the light receiving part 3 in this state.