An internal combustion engine controller comprising: an oxygen concentration sensor outputting an electric signal having a value depending on an oxygen concentration in an exhaust gas flowing through an exhaust passage of an internal combustion engine; and a control unit controlling a fuel injection amount depending on at least said electric signal, said control unit being capable of performing atmospheric learning to calibrate said oxygen concentration sensor; characterized in that wherein said control unit is configured to perform said atmospheric learning when a total volume of intake air sucked in and supplied to said engine since a time of start of cutoff of fuel supply to said engine exceeds a predetermined threshold volume, wherein said control unit includes: a function of calculating an integrated flow of said intake air since the time of said start; and a function of determining whether or not said calculated integrated flow exceeds said predetermined threshold volume, a function of measuring a passed time since the time of said start; a function of calculating an average flow rate of said intake air after the time of said start by dividing said calculated integrated flow by said measured passed time; a function of calculating an arriving time needed for a gas within a cylinder of said engine to arrive at said oxygen concentration sensor from the time of said start on the basis of said calculated average flow rate by referring to a map defining a relationship between the average flow rate and the arriving time; a function of making, upon detecting that said measured passed time exceeds said calculated arriving time, an assumption that said total volume of said intake air exceeds said predetermined threshold volume, wherein said control unit is configured to correct said calculated integrated flow on the basis of at least one of a temperature and a pressure of said exhaust gas flowing through said exhaust passage. an oxygen concentration sensor outputting an electric signal having a value depending on an oxygen concentration in an exhaust gas flowing through an exhaust passage of an internal combustion engine; and a control unit controlling a fuel injection amount depending on at least said electric signal, said control unit being capable of performing atmospheric learning to calibrate said oxygen concentration sensor; characterized in that wherein said control unit is configured to perform said atmospheric learning when a total volume of intake air sucked in and supplied to said engine since a time of start of cutoff of fuel supply to said engine exceeds a predetermined threshold volume, wherein said control unit includes: a function of calculating an integrated flow of said intake air since the time of said start; and a function of determining whether or not said calculated integrated flow exceeds said predetermined threshold volume, a function of calculating an integrated flow of said intake air since the time of said start; and a function of determining whether or not said calculated integrated flow exceeds said predetermined threshold volume, a function of measuring a passed time since the time of said start; a function of calculating an average flow rate of said intake air after the time of said start by dividing said calculated integrated flow by said measured passed time; a function of calculating an arriving time needed for a gas within a cylinder of said engine to arrive at said oxygen concentration sensor from the time of said start on the basis of said calculated average flow rate by referring to a map defining a relationship between the average flow rate and the arriving time; a function of making, upon detecting that said measured passed time exceeds said calculated arriving time, an assumption that said total volume of said intake air exceeds said predetermined threshold volume, wherein said control unit is configured to correct said calculated integrated flow on the basis of at least one of a temperature and a pressure of said exhaust gas flowing through said exhaust passage.