An indoor installation type combustion apparatus having an exterior case and, housed therein: a burner; a combustion box for enclosing a combustion space of air-fuel mixture ejected from the burner; and an exhaust duct through which exhaust gas discharged from the combustion box flows, wherein an upper surface of the exterior case has formed therein: a cylindrical exhaust connection port which is in communication with the exhaust duct and which is connectable to an exhaust pipe; a first cylindrical air supply connection port which is connectable to an air supply pipe of double-pipe system containing therein the exhaust pipe and which encloses the exhaust connection port; and a second cylindrical air supply connection port which is connectable to an air supply pipe of twin-pipe system which is separate from, and independent of, the exhaust pipe, characterized in: that an air supply box which is in communication with both the first and the second air supply connection ports is disposed; that an outlet opening is formed at one place of the air supply box so that the air flowing in from an arbitrary air supply connection port between the first and the second air supply connection ports is discharged into an inner space of the exterior case; and that a supply-air filter to collect foreign matters in the air is mounted in the outlet opening. a burner; a combustion box for enclosing a combustion space of air-fuel mixture ejected from the burner; and an exhaust duct through which exhaust gas discharged from the combustion box flows, wherein an upper surface of the exterior case has formed therein: a cylindrical exhaust connection port which is in communication with the exhaust duct and which is connectable to an exhaust pipe; a first cylindrical air supply connection port which is connectable to an air supply pipe of double-pipe system containing therein the exhaust pipe and which encloses the exhaust connection port; and a second cylindrical air supply connection port which is connectable to an air supply pipe of twin-pipe system which is separate from, and independent of, the exhaust pipe, a cylindrical exhaust connection port which is in communication with the exhaust duct and which is connectable to an exhaust pipe; a first cylindrical air supply connection port which is connectable to an air supply pipe of double-pipe system containing therein the exhaust pipe and which encloses the exhaust connection port; and a second cylindrical air supply connection port which is connectable to an air supply pipe of twin-pipe system which is separate from, and independent of, the exhaust pipe, characterized in: that an air supply box which is in communication with both the first and the second air supply connection ports is disposed; that an outlet opening is formed at one place of the air supply box so that the air flowing in from an arbitrary air supply connection port between the first and the second air supply connection ports is discharged into an inner space of the exterior case; and that a supply-air filter to collect foreign matters in the air is mounted in the outlet opening. that an air supply box which is in communication with both the first and the second air supply connection ports is disposed; that an outlet opening is formed at one place of the air supply box so that the air flowing in from an arbitrary air supply connection port between the first and the second air supply connection ports is discharged into an inner space of the exterior case; and that a supply-air filter to collect foreign matters in the air is mounted in the outlet opening.