PURPOSE: To make it possible to inexpensively detect a flow amount with good accuracy, by mounting a phase difference detector for detecting the phase difference between the output of a wave form shaping device and a rectangular wave signal.

CONSTITUTION: A heat-generating element 2 is heated by the output of the heat- generating element heater 12 controlled by a rectangular wave signal generator 11, that is, power continuously and repeatedly changing in a rectangular wave form and cooled by the fluid 8 to be measured flowing through a sensor case 10 to the direction shown by an arrow. A heat-receiving element 9 is heated by the fluid 8 heated by the heat-generating element 2 and the temp. of the heat-generating element 2 also changes continuously and repeatedly in a rectangular wave form. Therefore, the temp. of the heat-receiving element 9 comes to a rectangular wave form accompanied by the wasteful time in inverse proportion to the flow amount of the fluid 8, that is, phase difference. The electric signal of the change in the temp. of the heat receiving element 9 receives wave form shaping by a wave form shaping device 13 and, therefore, by detecting the phase difference between the rectangular wave output of the wave form shaping device 13 and that of a generator 11 by a phase difference detector 14, the flow amount of the fluid 8 can be detected.