Flow sensor

A flow sensor, comprising a heating means (11) heating the partial area of the outer wall surface of a pipe forming a flow passage or a specified area inside the pipe and a temperature detection means measuring temperatures in areas on the upstream side and downstream side of the pipe in a heated area where the effect of temperatures from the heated area is radiated as heat energy (infrared) to the outside. The temperature detection means further comprises an upstream side non-contact temperature detection means (12) and a downstream side non-contact temperature detection means (13) disposed near the outer wall surface of the pipe in the state of non-contact with the outer wall surface. Each temperature detection means can measure a temperature in an upstream side heat energy radiation area and a temperature in a downstream side heat energy radiation area in the state of non-contact with the outer wall surface of the pipe. The flow rate of a measured fluid flowing in the flow passage is measured by using a difference between the temperature in the upstream side heat energy radiation area and the temperature in the downstream side heat energy radiation area measured by the non-contact temperature detection means or an output developed due to the temperature difference.