A method of operating an air-supplied type coating booth adapted to enforce conditioned air by an air supply blower through a filter downwardly to the inside of a tunnel-shaped coating booth at a predetermined flow velocity and then drawn to discharge together with coating mists, etc. to the beneath of the floor comprises the step of detecting the flow velocity of conditioned air flowing through the filter to the inside of continuously, applying primary and secondary averaging processings for the detected signals containing variations fluctuating at relatively shorter and longer periods respectively, comparing the value obtained from the signals applied with the secondary averaging processing with a predetermined flow velocity value and variably controlling the flow rate of the air supplied from the air supply blower depending on the deviation and thereby maintaining the flow velocity of the conditioned air flowing to the inside of the coating booth always constant. Reduction of the flow velocity supplied to the inside of the air-supplied type coating booth due to the filter clogging can be compensated exactly with no effects of external disturbances.