In a flow sensor, an upstream heating element and a downstream heating element are disposed, respectively, upstream and downstream in a flow direction of a fluid so as to undergo mutual thermal interference. By controlling heating currents flown through the respective heating elements, an average temperature control portion maintains an average temperature of the two heating elements at a temperature predetermined degrees higher than a temperature of the fluid detected by a fluid temperature detection portion while a voltage ratio control portion performs control so that a temperature difference or a ratio of applied voltages between the two heating elements takes a predetermined value. A detection signal corresponding to a flow direction of the fluid is obtained from a state of the control on the heating currents and whether a flow direction of the fluid is forward or backward is determined.