PURPOSE: To perform operation in an optimum state by detecting the rates of external air flow, circulating air flow and exhaust air flow respectively by sensors and controlling the air quantity of a main blower and exhaust blower thereby controlling cleanliness and degree of negative pressure.

CONSTITUTION: The invention relates to the safety cabinet to perform experiment and operation having the possibility of causing hazards on living things, etc., in which the rate of the external air flow intruding into the cabinet from an inlet and outlet port 4, the rate of the circulating air flow by the main blower 10 and the rate of the exhaust air flow by the exhaust blower 12 are detected, respectively, by air flow rate sensors 15. The signals thereof are inputted to a control device 16 for blower drive and the compared with the set values from a set value circuit 18 in a comparator 19. The respective air flow rates are judged by a judgement circuit 20. The motor rotating speeds of the blowers 10, 12 are controlled in a motor control circuit 22 by the commands thereof. The working space in the cabinet is thus maintained always at the set cleanliness and degree of negative.