A control system for an internal combustion engine, the internal combustion engine including a limiting current sensor that is configured to detect a concentration of oxygen in exhaust gas, the control system comprising: an electronic control unit configured to: (i) set a voltage applied to the limiting current sensor to an ordinary voltage for measuring the concentration of oxygen; (ii) control the internal combustion engine such that the concentration of oxygen in exhaust gas is kept at a constant value; (iii) execute a sweep process for gradually reducing the voltage from a first voltage which is greater than the ordinary voltage to a second voltage which is equal to or greater than the ordinary voltage, sulfur being produced as a result of reduction of SOx in the limiting current sensor and being accumulated in the limiting current sensor at the first voltage, sulfur in the limiting current sensor being oxidized into SOx at the second voltage; (iv) acquire an extreme value (Ip) of an output current of the limiting current sensor during execution of the sweep process from output currents of the limiting current sensor while a voltage included in a specific voltage range is applied to the limiting current sensor, the extreme value (Ip) being predicted to be output based on the concentration of oxygen having the constant value in the specific voltage range; and (v) detect the concentration of SOx in exhaust gas based on the extreme value (Ip) and a reference value (Iref), the reference value (Iref) being a value of limiting current of the limiting current sensor, the value of limiting current of the limiting current sensor corresponding to the concentration of oxygen having the constant value, said concentration of oxygen being measured when the voltage applied to the limiting current sensor is set to the ordinary voltage. (i) set a voltage applied to the limiting current sensor to an ordinary voltage for measuring the concentration of oxygen; (ii) control the internal combustion engine such that the concentration of oxygen in exhaust gas is kept at a constant value; (iii) execute a sweep process for gradually reducing the voltage from a first voltage which is greater than the ordinary voltage to a second voltage which is equal to or greater than the ordinary voltage, sulfur being produced as a result of reduction of SOx in the limiting current sensor and being accumulated in the limiting current sensor at the first voltage, sulfur in the limiting current sensor being oxidized into SOx at the second voltage; (iv) acquire an extreme value (Ip) of an output current of the limiting current sensor during execution of the sweep process from output currents of the limiting current sensor while a voltage included in a specific voltage range is applied to the limiting current sensor, the extreme value (Ip) being predicted to be output based on the concentration of oxygen having the constant value in the specific voltage range; and (v) detect the concentration of SOx in exhaust gas based on the extreme value (Ip) and a reference value (Iref), the reference value (Iref) being a value of limiting current of the limiting current sensor, the value of limiting current of the limiting current sensor corresponding to the concentration of oxygen having the constant value, said concentration of oxygen being measured when the voltage applied to the limiting current sensor is set to the ordinary voltage.