In order to explore the deep space, such as Mars, Jupiter, Saturn, etc in the future, a spacecraft that will be driven by nuclear power should be developed. At present, satellites or space probes have been using mainly electric source of chemical battery, fuel battery, solar battery, and RI battery. However, considering highly developed and extensive space exploration in the future, it is obvious that larger electric power is required over the long term space travel more than several years. Additionally, the solar battery used in space will be fundamentally impossible to use in planetary exploration father away form Mars because sunlight is attenuated. Therefore, lager electric power source must be installed in the space craft. In this study, we consider about co-generation system for heat and electricity using nuclear power. We think that the nuclear power is appropriate for using in deep space because of a long time operation without refueling and possibility in downsizing due to higher power density. We selected the SMR type fast reactor system of about 18 MWth compared with other type of reactors, such as PWR and high temperature gas reactor. With regard to a power generation system, we examined about efficiency of Stirling engine compared with a gas-turbine engine. Theoretical efficiency of Stirling engine is much higher than that of gas-turbine engine. Therefore, we selected Stirling engine and we have started the model test of a Stirling engine. Total power generation at International Space Station (ISS) that has been built since 1998 is about 110kWe. We estimated that about 5times as much electricity as that of ISS is enough to explore or develop the space. In that case, 2.5MWe will be generated by the system, number of crews will be about 5 and 2MW will be used to electric propulsion.