PURPOSE: To provide a highly reliabble thermostat of a simple structure and small occupation volume by inspecting the thermostat in its manufacture process despite of having a built-in fuse spring impossible to make repetitive action.

CONSTITUTION: In the manufacture of a fuse spring built-in thermostat with a built-in fuse spring 8 consisting of a spring 8a fixed by an easy-melt alloy or synthetic resin 8b in its deformed state, the fuse spring 8 part and other basic thermostat part are independently manufactured. Impossibility of repetitive action of the spring 8 results in a sampling inspection at every manufacture lot in a sampling inspection process. In a built-in process, the spring 8 having passed the inspection is built in the basic thermostat part having passed the inspection. After that, the sampling and outward appearance inspection of the completed article is performed before its shipment.