A flow sensor may include a heater for heating a partial region of the outer wall surface of a pipe forming a flow path or for heating a particular region in the pipe; and a temperature detector for measuring the temperatures of regions, the regions being on the upstream side and the downstream side of the pipe with respect to the heated region. The temperature detector may include an upstream-side non-contact temperature detector and a downstream-side non-contact temperature detector arranged near the outer wall surface of the pipe in a state of being not in contact with the outer wall surface, and the non-contact temperature detectors can measure the temperatures of the upstream-side heat energy radiating region and downstream-side heat energy radiating region in a state of being not in contact with the outer wall surface of the pipe.