Abstract: An object of the present invention to provide a gas supply device, a gas supply device having a mixing function, a welding device, and a gas supply method, which can prevent rush flow of gas from the gas supply source in the initial stage of gas lead-out from a gas supply source, reduce the cost, and improve the maintenance; and the present invention provides a gas supply device including a gas supply source into which a gas is filled at a high pressure, a single-stage decompressor which is provided at a gas outlet of the gas supply source and which is configured to reduce pressure of a gas led out from the gas supply source to a predetermined pressure, a gas supply line having one end connected to the single-stage decompressor and the other end connected to a use destination of the gas, a solenoid valve provided in the gas supply line, and a flow regulating valve provided in the gas supply line located between the solenoid valve and the use destination.