Thermal flow rate sensor supplying digital output

The invention discloses a thermal flow rate sensor adapted to measure a flow rate of a fluid, and comprises: a bridge circuit (100) including a heat-generating resistor (1) generating heat by receiving a supplied current, a first temperature detector (2) heated by the heat-generating resistor (1) and having a resistance value in accordance with the temperature of the heat-generating resistor (1), and a second temperature detector (3) not heated by the heat-generating resistor (1) and having a resistance value varying in accordance with a temperature of the fluid to be measured; a comparison portion (81) adapted to compare a voltage at a portion other than both end portions of the first temperature detector (2) and a voltage at a portion other than both end portions of the second temperature detector (3), and to supply a digital output indicating a result of voltage comparison; a voltage-controlled current source (15) adapted to determine a value of the current based on the digital output received from the comparison portion (81) and indicating the result of comparison and regardless of variation in the resistance value of the heat-generating resistor (1), and to supply the current to the heat-generating resistor (1); and an output operation portion (82) adapted to operate a flow rate for a prescribed period, of the fluid which is an object of measurement based on the digital output indicating the result of comparison for the prescribed period, and to supply a digital output indicating a result of operation.