To provide a mass flowmeter preventing measuring signals from temporarily decreasing when flow within a flow pipe is weak.

The thermal mass flowmeter is provided with a flow pipe having a temperature sensor in an upstream position A and a heater H in a downstream position B and through which fluid flows during operation; and a flow sensor having a power control means and a temperature measuring means both for keeping at a preset value the temperature difference between the positions A and B. The flowmeter has a means provided between the positions A and B for preventing the temperature at the position A from rising due to heat conduction from position B to position A via the pipe when there is no flow or when flow is weak.