The utility model relates to a flow sensor by which an error caused by dust attached to the flow sensor can be accurately corrected, and the output characteristic of the relation between the temperature measured by a temperature measuring unit and the flow rate of the fluid is changed due to the dust attachment, etc. in the flow sensor. A film-shaped bridge part is formed on the surface of a basal plate and supported on the gap part of the basla plate in an overhead mode. A heater for heat generation and the temperature measuring unit are arranged on the surface of the bridge part. The relation (output characteristic) between the temperature measured by the temperature measuring unit in the initial state and the flow rate of the fluid to be measured is stored in a memory of a counting processing part. After the flow sensor detects the temperature measured by the temperature measuring unit with the flow rate of zero during the operation, the ratio of the value of the temperature measured by the temperature measuring unit with the flow rate of zero in the initial state to the temperature measured by the temperature measuring unit with the flow rate of zero during the operation is calculated, and the measured temperature ratio multiplies the output of the temperature measuring unit. The output of the temperature measuring unit is corrected. Then, the flow rate of the fluid is determined according to the corrected value of the output of the temperature measuring unit and the relation stored in the memory device.