Abstract: In a gas supply device supplying many different gases to a gas use portion through many flow rate controllers, a flow rate controller calibration unit includes a build-up tank with inner volume, an inlet side on-off valve and an outlet side on-off valve V2 of the tank, and a gas pressure detector and a gas temperature detector for gas inside the tank, joined in a branched form to a gas supply line, with the valve V2 connected to vacuum. The calibration unit is used to calibrate a flow rate controller based on performing a first measurement of gas temperature and pressure inside the tank, and then building-up gas into the tank, and then performing a second measurement of gas temperature and pressure, and from respective measured values, calculating gas flow rate Q and by comparing a set gas flow rate and calculated gas flow rate Q, performing flow rate calibration.