To simplify assembling work so as to prevent generation of defective insertion and the like by inserting a thermostat into a thermocase in which a connector and a lead wire communicating with a control unit are connected together and installing this set of parts on a combustion chamber cover by means of machine screws.

Inside a thermocase B16, connectors 19, 20 are connected to lead wires 21, 22 communicating with a control unit. Terminals 14, 15 of a thermostat B13 are inserted along tapered end faces 17, 18 of the thermocase B16 so as to be buried in the connectors 19, 20. Then, this set of parts is installed on a combustion chamber cover 1 by means of machine screws 24, 25, and the temperature of the combustion chamber cover 1 is controlled by means of thermostat B13. In this way, assembly can be manually carried out with ease, wiring work can be greatly improved, and coming off, loosening, or breakage of a wiring line due to vibration and the like can be reduced.