In a method according to the present invention for calculating the combustion ratio RC of fuel added to an exhaust passage, the temperature To of the exhaust gas flowing in the exhaust passage located downstream of a region in which the fuel added to the exhaust passage is burned is acquired; the thermal capacity CE of the exhaust gas is acquired; the energy quantity QF of the fuel supplied to the exhaust passage is acquired; an increase ΔTo in exhaust temperature To immediately before the exhaust gas is heated is obtained; a change ratio dTo of the exhaust temperature To immediately before the exhaust gas is heated is obtained; a parameter Z relevant to the exhaust gas flowing in the exhaust passage is acquired; and the combustion ratio RC of the fuel added to the exhaust passage is calculated in accordance with RC=(CE/QF)(Z·dTo+ΔTo).