PURPOSE: To eliminate malfunctioning of a gas sensor even with emanation of an aromatic by invalidating a reading of an output value of a gas sensor while scent is emanating after released by a scent releasing means for releasing scent intermittently.

CONSTITUTION: Cycle is set to release scent with a cycle setting device 11 to open a scent path 14 and then, an erroneous detection time during which a gas sensor 7 is affected by an aromatic is set to T2. Now, when time T1 during which the scent path 14 is open is short and the time T2 is short enough as compared with a period during which the gas sensor senses smoke of tobacco by smoking, an output Rc(t) of the gas sensor 7 decreases to t=T1 from t=0 and increases as t=T1 is passed. A normal smoke density detection level X is set lower slightly as compared with an output level Rc(0) of the gas sensor 7 in a clean air, but the output Rc(t) of the gas sensor 7 is down below X until the time T2 elapses. A control section 9 operates to ignore an output value of the gas sensor 7 during a period of time 0-T2.