An intake air volume measuring device for an internal combustion engine, said device comprising: an air flow meter adapted to be located in an intake air path of the internal combustion engine, to measure a volume of intake air sucked into a combustion chamber of the internal combustion engine based on an output signal of the air flow meter; and a control unit configured to sample the output signal of the air flow meter in synchronism with a crank angle of the internal combustion engine at predetermined crank angle intervals to obtain sets of data, calculate an average of the sets of data to obtain an average flow rate of the intake air, switch the crank angle intervals according to the number of engine revolutions of the internal combustion engine, and set the crank angle intervals to a greater value for a relatively large number of engine revolutions of the internal combustion engine than for a relatively small number of engine revolutions of the internal combustion engine, characterized in that the control unit is further configured to: when the crank angle intervals are to be switched, if the crank angle intervals have a pre-switching value and a post-switching value such that either one of the pre-switching and post-switching values is a multiple of the other, calculate the average using sampling values obtained with a greater one of the pre-switching and post-switching values, and, if the crank angle intervals have a pre-switching value and a post-switching value such that neither one of the pre-switching and post-switching values is a multiple of the other, calculate the average using (i) a value obtained by linear interpolation of two sampling values obtained with the pre-switching value of the crank angle intervals and (ii) a sampling value obtained with the post-switching value of the crank angel intervals. an air flow meter adapted to be located in an intake air path of the internal combustion engine, to measure a volume of intake air sucked into a combustion chamber of the internal combustion engine based on an output signal of the air flow meter; and a control unit configured to sample the output signal of the air flow meter in synchronism with a crank angle of the internal combustion engine at predetermined crank angle intervals to obtain sets of data, calculate an average of the sets of data to obtain an average flow rate of the intake air, switch the crank angle intervals according to the number of engine revolutions of the internal combustion engine, and set the crank angle intervals to a greater value for a relatively large number of engine revolutions of the internal combustion engine than for a relatively small number of engine revolutions of the internal combustion engine, characterized in that the control unit is further configured to: when the crank angle intervals are to be switched, if the crank angle intervals have a pre-switching value and a post-switching value such that either one of the pre-switching and post-switching values is a multiple of the other, calculate the average using sampling values obtained with a greater one of the pre-switching and post-switching values, and, if the crank angle intervals have a pre-switching value and a post-switching value such that neither one of the pre-switching and post-switching values is a multiple of the other, calculate the average using (i) a value obtained by linear interpolation of two sampling values obtained with the pre-switching value of the crank angle intervals and (ii) a sampling value obtained with the post-switching value of the crank angel intervals.