Cylinders of an internal combustion engine are divided into at least two cylinder groups. First air-fuel ratio sensors are disposed in each exhaust branch pipe connected to the cylinder groups, and a second air-fuel ratio sensor is disposed in a common exhaust pipe upstream from the catalyst. When a vapor amount introduced into an intake passage during purge control is determined, the vapor amount is determined during normal operation using output values from the first air-fuel ratio sensors and the vapor amount value learned during normal operation. During rich-lean operation, the vapor amount is determined using the output value from the second air-fuel ratio sensor and the vapor amount value learned during rich-lean operation. Thereby, the vapor amount introduced into the intake passage when a switch is made from normal to rich-lean operation of the internal combustion engine is accurately determined.