To provide a gas supply system, capable of obviating malfunctions of a thin film forming device by automatically suspending film deposition processing according to the change in the flow rate of an additive gas.

The gas supply system comprises a material gas supply system and an additive gas supply system. The material gas supply system comprises an accumulator for accommodating liquid reaction materials, a first piping for transmitting a vaporized material gas, a first mass flowrate controller located above the first piping, and a pressure detector downstream of the first mass flowrate controller. The first gas flowrate controller includes a flowrate control bulb in the upstream region, and a sonic nozzle type flow rate detector in the downstream region. The additive gas supply system comprises at least one additive gas inflow port, at least one second piping, and at least one second mass flow rate controller above the second piping. The first piping and second piping merge on the upstream of a reaction chamber, and the pressure detector and a controlling device are electrically connected.