Abstract: A flow rate detector mechanism using variable Venturi therein, comprising: a variable flow rate generator, comprising: a core 11; and a variable Venturi 12; wherein a throat (flow passage) cross-sectional area defined between the core and the venturi is able to be changed by shifting relative positions of the core and the venturi in a direction of axes thereof, and further comprising a flow rate calculation processing portion 30 for calculating a flow rate based on the relative positions in the direction of the axes thereof and for outputting the calculated flow rate, thereby continuously changing the constant flow rate, without occurrence of any disturbance therein.