A semiconductor type flow rate detecting apparatus having first and second control units arranged at upstream and downstream positions respectively in a flow stream to be measured and an operation circuit. The first control unit includes a first temperature detecting element and a first temperature control heater. The second control unit includes a second temperature detecting element, a second temperature control heater and a preliminary heater. The operation circuit controls the second and preliminary heaters and generates a signal indicative of flow rate. The operation circuit has a constant power consumption circuit to control heat from the preliminary heater to a constant level.