PURPOSE: To measure flow rates and average flow speed highly accurately, without disturbing the flow of a fluid to be measured, by arranging a semiconductor flow rate sensor and a temperature detecting sensor on the wall of a pipe.

CONSTITUTION: A base voltage V6 of a heating power transistor Q3 is controlled, and a collector current ic3 is adjusted. Then the temperature of a chip 51 is increased. At this time, since the temperature of a transistor Q1 becomes the same as that of a transistor Q2, emitter-base voltages VBE1 and VBE2 become the same, emitter currents iE1 and iE2 become the same, and ic1 and ic2 become the same. When there is no flow, the output becomes zero. When a fluid flows along a carrier plate, a temperature boundary layer is formed. The temperature difference yielded between Q1 and Q2 gives effect on the emitter-base voltages VBE1 and VBE2 of the temperature checking transistors Q1 and Q2. With the change in iE1 and iE2, difference occurs between ic1 and ic2, and finally difference occurs between V01 and V02.