An object of the invention is to provide a technology pertaining to a control system for an internal combustion engine using CNG to allow an internal combustion engine to operate appropriately even when properties of CNG change. To achieve the object, in the control system for an internal combustion engine using compressed natural gas according to the invention, when air-fuel ratio feedback control that corrects the fuel injection quantity in such a way as to make the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine substantially equal to a target air-fuel ratio, a control parameter relating to a condition of combustion of the air-fuel mixture is corrected based on the magnitude of the correction value in the air-fuel ratio feedback control.