A micro flow sensor includes: a flow path through which a fluid to be measured flows; a detection unit having a heater configured to heat the fluid to be measured in the flow path and temperature sensors configured to measure temperature of a fluid to be measured in the flow path; an arithmetic unit configured to measure a flow rate of the fluid to be measured flowing through the flow path based on the temperature of the fluid to be measured, measured by the temperature sensors; and a narrowed portion disposed on an upstream side of the detection unit in the flow path for narrowing the flow path.