A thermal flow rate sensor adapted to measure a flow rate of a fluid, comprising: - a bridge circuit including a heat-generating resistor generating heat by receiving a supplied current, a first temperature detector heated by the heat-generating resistor and having a resistance value varying in accordance with the temperature of the heat-generating resistor, and a second temperature detector not heated by the heat-generating resistor and having a resistance value varying in accordance with a temperature of the fluid to be measured; - a comparison portion adapted to compare a voltage at a portion other than both end portions of the first temperature detector and a voltage at a portion other than both end portions of the second temperature detector, and to supply a digital output indicating a result of voltage comparison; - a voltage-controlled current source adapted to determine, regardless of variation in the resistance value of the heat-generating resistor, a value of the current based on the digital output received from the comparison portion and indicating the result of comparison and to supply the current to the heat-generating resistor; - a clock generator adapted to generate a clock; - an output operation portion adapted to operate a flow rate for a prescribed period by counting the clock generated by the clock generator, 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, wherein the output operation portion is adapted to calculate the sum of time periods during which an amount of current supplied to the heat-generating resistor is equal to or larger than a prescribed value in the prescribed period, based on the digital output indicating the result of comparison for the prescribed period, and to output, as a digital output, the sum of time periods as the flow rate for the prescribed period, of the fluid which is the object of measurement. - a bridge circuit including a heat-generating resistor generating heat by receiving a supplied current, a first temperature detector heated by the heat-generating resistor and having a resistance value varying in accordance with the temperature of the heat-generating resistor, and a second temperature detector not heated by the heat-generating resistor and having a resistance value varying in accordance with a temperature of the fluid to be measured; - a comparison portion adapted to compare a voltage at a portion other than both end portions of the first temperature detector and a voltage at a portion other than both end portions of the second temperature detector, and to supply a digital output indicating a result of voltage comparison; - a voltage-controlled current source adapted to determine, regardless of variation in the resistance value of the heat-generating resistor, a value of the current based on the digital output received from the comparison portion and indicating the result of comparison and to supply the current to the heat-generating resistor; - a clock generator adapted to generate a clock; - an output operation portion adapted to operate a flow rate for a prescribed period by counting the clock generated by the clock generator, 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,