A power system of the invention includes fuel cells and a fuel gas generation system that generates a fuel gas to be supplied to the fuel cells. At the time of stopping supply of hydrogen, the fuel gas generation system selectively uses a stop process that replaces hydrogen in a hydrogen separator unit with the air for removal of hydrogen and a pause process that allows hydrogen to remain in the hydrogen separator unit. The stop process is selected when the fuel gas generation system stops the supply of hydrogen for a long time period. The pause process is selected when the fuel gas generation system temporarily stops the supply of hydrogen. The arrangement of the invention desirably shortens a restart time of the fuel gas generation system and reduces a potential energy loss.