Abstract: In an apparatus in which a smoke density detecting element detects a smoke density in a room to be ventilated so as to automatically start and stop the operation of a ventilating fan in dependence upon whether the smoke density is higher than a predetermined value or not, if a difference between an actual output from the smoke density detecting element and an initial value thereof exceeds a set value, the operation of the ventilating fan is started, but if the difference therebetween becomes lower than the set value, the operation of the ventilating fan is stopped so as to absorb nonuniform measuring accuracy by the smoke density, and further, if the difference therebetween does not become higher or lower than the set value even though a predetermined time elapses after the operation of the ventilating fan is started or stopped, the set value is changed so as to cope with the atmosphere of the inside of the room to the ventilated.