A fluid mixing device is productive, maintainable and improved in operability and reliability even without high accuracy pressure control. A fluid mixing device includes a mixing tank, supply tanks, mixing tank flow channels, valves, and branch flow channels. The mixing tank mixes at least two liquids. Each supply tank is provided for each of the liquids and supplies each liquid to the mixing tank. Each mixing tank flow channel connects each supply tank to the mixing tank. Each valve is arranged in each of the mixing tank flow channels. Each branch flow channel is connected to each of the mixing tank flow channels and is connected from the one mixing tank flow channel to the valve in the other mixing tank flow channel. Further, when the valve has pressure applied thereto from the liquid passing through the branch flow channel, the valve closes the mixing tank flow channel.