NOVELTY - A fuel cell system comprises combustor (10) which has burner used during start of fuel cell main body (2) and during electric power generation of fuel cell main body, starting fuel line (29) which supplies starting fuel to combustor, off gas fuel line (30) which supplies off gas fuel to combustor, process gas line (42), off gas fuel line non-return valve (24) which cuts starting fuel line and off gas fuel line before electric power generation of fuel cell main body, and deaeration line (17) which has 1 end connected to process gas line of fuel cell main body.    USE - A fuel cell system for power source system of residences.    ADVANTAGE - Ignition quality and combustion stability during electric power generation can be improved, and fuel cell system with high electric power generation efficiency can be provided.    DETAILED DESCRIPTION - A fuel cell system comprises combustor (10) which has burner used during start of fuel cell main body (2) and during electric power generation of fuel cell main body, starting fuel line (29) which supplies starting fuel to combustor, off gas fuel line (30) which supplies off gas fuel to combustor, process gas line (42), off gas fuel line non-return valve (24) which cuts starting fuel line and off gas fuel line before electric power generation of fuel cell main body, deaeration line (17) which has 1 end connected to process gas line of fuel cell main body and other end connected to exhaust gas side, and control unit (41) which deaerates inert gas existing in process gas line before electric power generation of fuel cell main body through deaeration line and closes deaeration line when reformed fuel is supplied to process gas line. Starting fuel line and off gas fuel line are connected. Reformed fuel from fuel modifier (6) is transmitted to fuel electrode of fuel cell main body.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the fuel cell system. (Drawing includes non-English language text).    Fuel cell main body (2)    Fuel modifier (6)    Combustor (10)    Deaeration line (17)    Off gas fuel line non-return valve (24)    Starting fuel line (29)    Off gas fuel line (30)    Control unit (41)    Process gas line (42)