To provide a fluid mass flow rate controller which never causes the clogging at a flow rate detection part, has high heat resistance and can control even a large flow rate with high accuracy.

A fluid mass flow rate controller 1 consists of an ultrasonic flowmeter 3 which measures the flow rate of gas flowing in a gas pipe 2, a flow rate control valve 4 which controls the flow rate of gas flowing in the pipe 2 and a valve drive control circuit 5 which compares the mass flow rate measured by the flowmeter 3 with the mass flow rate of the target gas and controls the opening degree of the valve 4 to secure the coincidence between both mass flow rates. In such a constitution, the clogging is never caused at a flow rate detection part with high heat resistance, being different from the case where a thermal flow rate sensor is used. In addition, even a large flow rate can be controlled with high accuracy and the pressure loss of a gas flow is never caused at the flow rate detection part.