A gas concentration measuring apparatus comprising: a gas concentration sensor which includes a solid electrolyte body to be exposed to an exhaust gas within an exhaust system of an internal combustion engine; a sensor control circuit (30, 50) adapted to apply a voltage to said gas concentration sensor and to sample a resulting flow of electric current through said gas concentration sensor to produce a gas concentration output as a function of a concentration of a given component of the exhaust gas; a switch (35, 55) adapted to establish a current flow line which extends between the gas concentration sensor and the sensor control circuit (30, 50) and through which the resulting flow of the electric current passes; and a correcting circuit adapted to sample a first and a second value of the gas concentration output when said sensor control circuit (30, 50) is placed in a first and a second condition in which the exhaust gas is in a first and a second atmosphere preselected to be different from each other, respectively, said correcting circuit being adapted to determine a first and a second error as functions of differences between the first value and a corresponding first normal reference value as expected to be attained by said sensor control circuit (30, 50) correctly when the exhaust gas is in the first atmosphere and between the second value and a corresponding second normal reference value as expected to be attained by said sensor control circuit (30, 50) correctly when the exhaust gas is in the second atmosphere to determine a correction factor (Kaf), said correcting circuit is adapted to correct a value of the gas concentration output of said sensor control circuit (30, 50) using the correction factor (Kaf) so as to compensate for the first and second errors, wherein the first atmosphere is established when the sensor control circuit (30, 50) is placed in a virtual stoichiometric air-fuel ratio measuring mode equivalent to an operation mode in which the sensor control circuit (30, 50) produces the gas concentration output when the exhaust gas results from burning of a stoichiometric mixture in the engine, the virtual stoichiometric air-fuel ratio measuring mode being entered by opening the switch (35, 55). a gas concentration sensor which includes a solid electrolyte body to be exposed to an exhaust gas within an exhaust system of an internal combustion engine; a sensor control circuit (30, 50) adapted to apply a voltage to said gas concentration sensor and to sample a resulting flow of electric current through said gas concentration sensor to produce a gas concentration output as a function of a concentration of a given component of the exhaust gas; a switch (35, 55) adapted to establish a current flow line which extends between the gas concentration sensor and the sensor control circuit (30, 50) and through which the resulting flow of the electric current passes; and a correcting circuit adapted to sample a first and a second value of the gas concentration output when said sensor control circuit (30, 50) is placed in a first and a second condition in which the exhaust gas is in a first and a second atmosphere preselected to be different from each other, respectively, said correcting circuit being adapted to determine a first and a second error as functions of differences between the first value and a corresponding first normal reference value as expected to be attained by said sensor control circuit (30, 50) correctly when the exhaust gas is in the first atmosphere and between the second value and a corresponding second normal reference value as expected to be attained by said sensor control circuit (30, 50) correctly when the exhaust gas is in the second atmosphere to determine a correction factor (Kaf), said correcting circuit is adapted to correct a value of the gas concentration output of said sensor control circuit (30, 50) using the correction factor (Kaf) so as to compensate for the first and second errors, wherein the first atmosphere is established when the sensor control circuit (30, 50) is placed in a virtual stoichiometric air-fuel ratio measuring mode equivalent to an operation mode in which the sensor control circuit (30, 50) produces the gas concentration output when the exhaust gas results from burning of a stoichiometric mixture in the engine, the virtual stoichiometric air-fuel ratio measuring mode being entered by opening the switch (35, 55).