To measure the mixing ratio of the mixture fluid of conductive fluid and insulating fluid at low cost and with high accuracy over the wide flow conditions.

This mixing ratio measuring device is equipped with a vortex generator generating Karman vortex 3; a pair of measuring electrodes 4a, 4b installed in positions downstream than the vortex generator and detecting the change of induced electromotive force generating when the Karman vortex 3 passes through a magnetic field; a pair of reference potential measuring electrodes 6a, 6b for measuring the potential difference which is an upstream reference and a downstream reference; a detecting circuit 5 for measuring a frequency output signal of the Karman vortex 3 by contracting the potential difference from the induced electromotive force; and in addition a mixing ratio computing part 11 for computing the mixing ratio by comparing a frequency output signal of the Karman vortex 3 measured when only the conductive fluid is made to flow as the fluid with a frequency output signal of the Karman vortex 3 measured when the mixture fluid of the conductive fluid and the insulating fluid 9 is made to flow as the fluid.