A mass flow controller has: a flow rate sensor section that measures the flow rate of a fluid and outputs a flow rate measurement signal indicating the measurement value; a flow rate control valve disposed upstream or downstream of the flow rate sensor section; a calculation section that calculates a feedback control value to the supplied to the flow rate control valve by performing at least a proportional calculation on the deviation of the flow rate measurement value indicated by the flow rate measurement signal from a flow rate setting value; and an opening control signal output section that generates an opening control signal based on the feedback control value and outputs the opening control signal to the flow rate control valve. The function used for calculating the gain value in the proportional calculation differs between a changing period and a stable period.