A filter monitoring system for a filter in a vehicle air conditioner includes a first temperature sensor to detect an air temperature of intake air drawn into an air conditioner case through the filter, a second temperature sensor to detect an evaporator temperature of the evaporator, and a processor coupled to the first and second temperature sensors and coupled to a memory. The processor is configured to calculate an actual change-rate of the evaporator temperature while a compressor is in operation, calculate an expected change-rate of the evaporator temperature based on the air temperature and operation levels of a blower and the compressor by using predetermined data with a properly functioning filter, calculate a degradation degree of the filter by comparing the actual change-rate to the expected change-rate, and output a signal indicating degradation of the filter upon determining that the degradation degree is out of an acceptable range.