To provide an air vent type thermal detector in which tight seal and lubrication of the outer peripheral surface of a movable shaft and a cock main body are improved to a switching cock by simple assembling work and structure.

Fire generation is detected by detecting expansion of the inside air of an air vent with a detecting part. The switching cock 1 for switching over a prescribed air path in the case of fire monitoring and in the case of various kind of tests is installed. The cock 1 is equipped with a first and a second nozzles PN1, PN2 for air vent which have air holes for interconnection with the air vents 2 and a nozzle TN for test. A fixing hole 31 for fixing pivotably a movable shaft 4 to the cock is installed to be interconnected with the nozzles PN1, PN2 and the nozzle TN. The movable shaft 4 is covered with sheet material 5 made of rubber and inserted in the fixing hole 31. Vent paths AP1, AP2 which are interconnected with the air holes by rotating operation are formed in the shaft 4. In the sheet material 5, auxiliary air paths 51 are formed corresponding to the air vents AP1, AP2.