To provide constitution and structure of an external charging device, capable of easily giving a temperature difference to a thermal power generating timepiece at a low cost, when charging the timepiece by adding heat from the outside.

A semiconductor, whose resistance value is increased as a rise temperature, and sharply increased at the Curie temperature or above, is used as a heating means of the external charging device for a thermal power generating instrument which generates power using the temperature difference. Particularly, a PTC thermistor having the Curie heater temperature of 30°C to 60°C is used. Further, natural air-cooling or a blower fan is used as its cooling means.