A mixed gas supply device includes a plurality of gas supply lines arranged in parallel that include flow rate control devices and outlet side switching valves, wherein gas outlets of respective outlet side switching valves communicate with a manifold, and another gas supply line at a position close to a mixed gas outlet of the manifold supplies a low flow rate gas, wherein an outlet side of the flow rate control device and an inlet side of the outlet side switching valve are hermetically connected via an outlet side connecting fitting of the flow rate control device and a mounting table having a gas passage, wherein a small hole portion is provided at a part of a flow passage at the outlet side connecting fitting and/or a flow passage, which makes the outlet side switching valve and a mixed gas flow passage in the manifold communicate with one another.