The invention relates to a device for and a method of informing replacement time of an air filter, more particularly, to a device for and a method of informing replacement time of an air filter, in which the accumulation amount of the dust accumulated in the air filter is calculated based on the factors such as a contamination degree of the air introduced into the air filter, air introduction time, and air introduction volume, and then when the accumulation amount of the total dust in the air filter exceeds a reference value, a driver is informed of the replacement time of the air filter, so that it is possible to replace the air filter at an optimum instance to thereby maintain the pleasant interior circumstance of a vehicle, to prevent the dissipation of the expense caused by the unnecessary early replacement of the air filter, and to prevent the interior air contamination of the vehicle due to the delay of the replacement of the air filter, and the like.