A temperature sensor in which a thermocouple is embedded in a front end of a center electrode of a spark plug, the front end of which is disposed in a combustion chamber of an internal engine, and including a digital-optical converter which is arranged to convert the potential of the thermocouple into an optical output representing the temperature of the spark plug. A connector is arranged to be removably connected to a receptacle in the digital-optical converter. The connector has a pair of contacts which are adapted to energize the digital-optical converter when the connector is electrically connected to the receptacle.