PURPOSE: To prevent erroneous detection of flow rate by calculating flow rate in terms of average cycle and using the average cycle of the previous computation when a large cycle associated with missings of a pulse.

CONSTITUTION: A sinusoidal output signal of a Karman's vortex sensor 31 is fed to a waveform shaping circuit 32, where said sinusoidal signal is converted into a square wave signal. The square wave signal is fed to an interrupt input of a CPU33, which executes an interrupt routine according to the rising of the square wave signal. The step 402 of the interrupt routine, a value TQFX is taken in from a cunter 35, which is reset. Then, at the step 403, TQFX≤K1.TQF (K; constant) is set and when TQFX>K1.TQF is met, missings are determined to occur in the output pulse of the waveform shaping circuit 32. Then, a shift is made directly to the step 405. At the step 405, the average cycle TQF is used to compute the air intake Q in terms of QK2/TQF. Then, the return is made.