A combined combustion device comprises: a plurality of combustion units (2) each having a burner (4) and an air supply fan (5); an exhaust collecting pipe (10) connecting the plurality of the combustion units (2) to each other; and a check valve (7) which opens by rotation of the air supply fan (5) and prevents backflow of combustion exhaust gas from the exhaust collecting pipe (10) into each of the combustion units (2), wherein when one or more of the combustion units (2) among the plurality of the combustion units (2) are in a combustion operation state and the other one or more of the combustion units (2) are continuously maintained in a non-combustion operation state for a predetermined reference stoppage time or longer, the air supply fans (5) of the other one or more of the combustion units (2) are rotated for a certain time.