a microcomputer - based control device for an air conditioner to which a replaceable air filter is attached includes a dc power supply section for converting an ac voltage to a dc voltage , an air - conditioning control unit connected to the dc power supply and including a microcomputer , a control unit resetting circuit for delivering an initial reset signal to the control unit when a voltage to be supplied to the control unit from the dc power supply section has risen to a value at which the control unit is operable , an operation section connected to the air - conditioning control unit for setting an operating condition of the control unit , a non - volatile memory connected to the control unit so that data of a cumulative air - conditioning operation time period stored in the control unit is written into and read out of the non - volatile memory , a display for displaying an image indicative of a replacing time of the air filter when the cumulated airconditioning operation time period has reached a predetermined time period , and a voltage drop detector connected between the control unit and the dc power supply section for detecting the output voltage of the dc power supply section dropped to a predetermined level or below to deliver a data write command signal to the control unit so that it is operated in response to the data write command signal to write the data into the non - volatile memory .