To easily and surely detect clogging of an air filter even in case of variation of the rotation speed or failure of a cooling fan, or variation of brightness of measuring environment.

This clogging detecting device 14 is provided with a color information acquiring part 15 measuring a surface color of the air filter 13, a color information converting part digitizing the color information measured by the color information acquiring part 15 with an L*a*b*color system; and a control part 18 detecting a clogging state of the air filter 13 from the information about color and saturation digitized by the color information converting part. The clogging detecting device 14 is preferably provided with a color sensor or a CCD camera as the color information acquiring part 15, a display part 17 displaying the clogging state of the air filter 13 detected by the control part 18, and an output part transmitting the clogging state of the air filter 13 detected by the control part 18 to the outside.