PURPOSE: An apparatus for calibrating a mass flow meter is provided to measure the flow rate of a calibration gas by sensing the volume of a calibration flow path which changes according to a flow path of a mass flow meter. CONSTITUTION: A calibration block is communicated with a flow path of a mass flow meter and has a calibration flow path in which a calibration gas flows. A first valve(120) is arranged in an entrance of the calibration flow path. A second valve(130) is arranged between a first valve and the mass flow meter and limits the first volume of the calibration flow path. A third valve is arranged between the exit of the calibration flow path and the mass flow meter and limits the second volume of the calibration flow path. A temperature sensor respectively measures the temperatures of calibration gases in the first volume and the second volume. A pressure sensor(160) respectively measures the pressures of the calibration gases in the first volume and the second volume.
