A slope Δt1HC in a linear area of sensor output characteristics for a mixed atmosphere of CO and THC and a slope Δt1NH in the linear area of the sensor output characteristics for NH3 are specified in advance at a time when a time t1 has elapsed since a start of use of an engine. In performing calibration of an NH3 sensor when a time t2 (greater than the time t1) has elapsed, a slope Δt2HC in the linear area of the sensor output characteristics for the mixed atmosphere is specified, a value Δt2NH is calculated from an equation Δt2NH=Δt2HC/(Δt1HC/Δt1NH), and the calculated value Δt2NH is determined as a new slope in the linear area of the sensor output characteristics for an NH3 gas.