The device for detecting the air-fuel ratio in an internal combustion engine comprising an air-fuel ratio sensor arranged in an exhaust system of the engine and passes an electric current when an electric voltage is applied thereto, an air-fuel ratio sensor circuit that applies the electric voltage to the sensor, detects the current and outputs a signal proportional to the magnitude of the detected current, and a memory for storing a conversion map for calculating the air-fuel ratio in the engine corresponding to the output of the air-fuel ratio sensor circuit by the use of a reference air-fuel ratio sensor and the reference air-fuel ratio sensor circuit. The device further comprises: a sensor for determining the sensor inactive state when the difference between a first coolant temperature at the last engine stopped time and a second coolant temperature at the next engine start up time is equal to or more than a determined value; and a map for calibrating an air-fuel ratio in the engine calculated from the conversion map stored in the memory in response to the output from the sensor circuit based on the error between the output data of the sensor circuit when the sensor is determined to be in an inactive state by the sensor and the output data from the sensor circuit corresponding to the stoichiometric air-fuel ratio in the engine that is calculated from the conversion map, thereby correctly and very precisely detecting the air-fuel ratio.