PURPOSE: To make it possible to detect the contamination degree of an air filter from the single side surface thereof and to move said air filter.

CONSTITUTION: A detection part 1 is moved along a guide 21 by a moving means 18 so as to be separated by a definite distance from an air filter 1 to be measured. The light from a light emitting element 4 is reflected from the air filter 1 to be incident to a light receiving element 5. The reflectivity of the air filter 1 changes by the amount of minute dust adhered and collected to said air filter 1. In order to collect reflected light with good accuracy, a compensation part 9, which has a small filter 2 having the same forming surface as the air filter 1 to be measured, the light emitting element 4 and the light receiving element 5, is mounted. The signals of measurement and the measuring result can be utilized as the start signal of an automatic replacement device in order to determine replacing order.