A microcomputer acquires first and second NH3 electromotive forces from a first NH3 detection section and a second NH3 detection section, and computes a NH3 concentration, which is the concentration of NH3 contained in exhaust gas. The first and second NH3 detection sections output the first and second NH3 electromotive forces which vary with both the concentrations of NH3 and a flammable gas contained in the exhaust gas. The microcomputer acquires a second pumping current from an NOx detection section and computes an NOx concentration, which is the concentration of NOx contained in the exhaust gas. The NOx detection section outputs the second pumping current which varies with the concentration of NOx and the concentration of NH3. The microcomputer determines whether or not the exhaust gas contains the flammable gas based on at least the first and second NH3 electromotive forces and the second pumping current.