To make an appropriate heater output by avoiding a correction lack caused by a variation of a temperature rising ratio of cooling water when a basic output value against a heater of an air/fuel ratio sensor is corrected based on a cooling water temperature.

Various kinds of parameters are read-in and whether or not a heater operation condition is effected is investigated (S1, S2). In the case where the heater operation condition is effected, a basic output value A set corresponding to an engine operation state, a cooling water temperature correction output value B and a water temperature correction output value C at the time of starting damped every at an engine predetermined rotation from an initial value based on the cooling water temperature at the time of starting of an engine are read-in (S4) in order to compensate a heater output which can not be compensated by a cooling water temperature correction output value B caused by a deviation of the cooling water temperature by a variation of a temperature rising ratio of cooling water caused by a use of heating and an actual exhaust temperature. The cooling water temperature correction output value B and the water temperature correction output value B and the water temperature correction output value C at the time of starting are added to the basic output value A and a heater output value D is calculated and outputted (S5, S6).