The invention provides an air-fuel ratio control system in which high precision can be applied to the air-fuel ratio feedback control of an engine by using a gas sensor mainly configured by an oxygen sensor that prevents the deterioration of the holding power of sealing material which enables high density filling with a small compressive load or the deterioration of the sealing material. The gas sensor such as the oxygen sensor is based upon a gas content detecting sensor that includes a gas content detecting element and a holder holding the gas content detecting element and that seals a measuring part of the gas content detecting element in the holder by a sealing part in which the sealing material is compressively filled, and has a characteristic that the sealing material is molded by mixed powder including plural species of forms of particles.