To correctly measure the flow rate by providing a correction means to correct the delay and non-linearity of a heat wire air flow meter and a backflow ratio calculating means, and obtaining the mean flow rate from the output and the backflow ratio of the heat wire air flow meter which is corrected in delay and non-linearity.

The output signal of a heat wire air flow meter 1 is inputted in a delay and non-linearity correcting means 7 and a waveform characteristic quantity extracting means 6 in a CPU to correct the delay and non-linearity of the heat wire air flow meter 1. Waveform characteristic quantity is obtained from the detected signal of a crank angle sensor 5 by the waveform characteristic quantity extracting means 6. After backflow ratio is obtained from backflow ratio calculation information 10 and the waveform characteristic quantity by a backflow ratio calculating means 8, the mean flow rate is operated from the output of the heat wire air flow meter 1 in which the delay and non-linearity from the delay and non-linearity correcting means 7 are corrected and the backflow ratio obtained by the backflow ratio calculating means 8 by a flow rate calculating means 9, and the air flow rate is obtained correctly.