A thermal mass flow meter and a thermal mass flow control device addresses a thermal siphon error, even if they are in a compact and inexpensive structure, without using a flow path converting block. A control computing process portion is configured to correct a measurement error caused by thermal siphon by calculating a correction value based on a measurement value at time of depressurizing fluid flow path and flow rate measuring conduit to an atmospheric pressure or less, a difference between the measurement value and a measurement value at time of charging an actual fluid into the flow rate measuring conduit, kind of the actual fluid, pressure at time of charging the actual fluid, and flow ratio of the fluid flowing in the fluid flow path and the flow rate measuring conduit, storing the correction value, and correcting an actual measured output flow value by the stored correction value.