A flow sensor measures an amount of flow of a medium flowing inside a tubular organ of an organism by mounting a flexible substrate, upon which heaters are formed, inside a tube disposed in a manner so as to follow the tubular organ of the organism through which the medium is flowing, and by detecting a state of heat quantity generated from the heaters on the mounted flexible substrate being transmitted to the medium. In the flow sensor, at least the two heaters, formed upon the flexible substrate, respectively form symmetrical structures with identical heating capabilities under identical conditions, and the linear members, which supply power to the heaters from outside the organism, form symmetrical structures with identical heating capabilities under identical conditions, thereby allowing temperature distribution of the heaters and linear members inside the tube to he symmetrical. The plurality of linear members protrude in a manner so as to intersect from inside the tubular organ to outside the organism.