PURPOSE: To make an air cleaner highly safe, reliable and useful by subjecting the output of a gas sensor detecting contamination of the air in a room to use a relative value as an input signal for automatic operation and an absolute value as an input signal for alarming.

CONSTITUTION: The apparatus is provided with a gas sensor 17 sensitive to kinds of gas components. An alarming means 23 gives an alarm to the user for contamination of the air in the room. Further, when the apparatus works automatically, a control unit 24 performs a program control corresponding to the change in the amount of the output of the gas sensor 17 from the value at a certain starting point, and after the output increase is stopped, the control unit sets a new starting point. The control unit further operates the alarming means 23 where the output of the gas sensor 17 has exceeded a predetermined absolute value for at least a certain period of time. As a result, when the air is badly contaminated, the apparatus gives an alarm for the contamination so that the user can notice the need for ventilation etc. Moreover, the detection of gas components remaining in the room and not affecting the human body can prevent useless operation from continuing.