An air-fuel ratio control apparatus estimates, on the basis of an exhaust air-fuel ratio of exhaust gas flowing into an exhaust purifying catalyst unit disposed in an exhaust passage of an engine, an emission of at least one specific component contained in exhaust gas flowing out of the exhaust purifying catalyst unit. The first RF antenna includes a primary coil provided on or above the dielectric window and electrically connected to the first RF power supply unit; and a secondary coil provided such that the coils are coupled with each other by an electromagnetic induction therebetween while being arranged closer to a bottom surface of the dielectric window than the primary coil. The elastic layers may serve to modify the neutral plane of the piezoelectric sense element.