PURPOSE: To effectively detect vent-up at-the initial stage without inconvenience such as the decrease or disabling of the detecting function by providing the detecting end of a heat radiation type flow switch in the vent chamber of an extruder. CONSTITUTION: An openable cover 6 is hinge-coupled to a vent chamber 5, and a transparent glass 7 is so mounted as to see through the chamber 5. A vacuum pump 9 for sucking gas from the chamber 5 is connected to the chamber 5 via a suction tube 8, and substantially always operated during the operating of an extruder 1. The detecting end 11 of a heat radiation type flow switch 10 is disposed in the chamber 5. The switch 10 is fixed at its body to the cover 6. A detection signal from the switch 10 is sent to an alarm unit 13 via a cable 12 to generate an alarm at the time of vent-up.