PURPOSE: To prevent accident due to malfunction of sequence, by actuating the control circuit of valve to close the valve when the filament of Pirani tube is broken.

CONSTITUTION: If a filament F1 of a Pirani tube 1 is broken while the valve is open, voltage VA at connection point A rises up to supply voltage 5 V. Therefore, the voltage VA becomes higher than the voltage VR at connection point R, so that a comparator IC1 supplied output voltage of logic 0 into the input terminal of NAND circuit IC2. The NAND circuit IC2 generates an output of about 5 V, and a transistor Tr1 becomes ON and a transistor Tr2 becomes OFF, thereby actuating a control circuit 4 to close the valve. Thus, accident due to malfunction of sequence may be prevented.