Abstract: A suction air measuring device comprises vortex detection means for detecting a Karman vortex signal of vortex in a suction conduit of an engine as a digital signal and storing it in a storage means at a predetermined time period and digitizing means for digitizing an output of the vortex detection means to obtain a Karman vortex pulse signal, setting or resetting the Karman vortex pulse signal when an output of the vortex detection means increases from a lower extreme thereof resetting or setting the Karman vortex signal when the output of the vortex detection means decreases from an upper extreme thereof by a predetermined value.