To reduce noises generated from an injector capable of adjusting flow rates of a fuel gas and a fuel off-gas supplied to a fuel cell.

A fuel gas supply device 1 for supplying a hydrogen gas to the fuel cell and circulating a hydrogen off-gas from the fuel cell to the fuel cell by mixing the hydrogen off-gas into a hydrogen gas includes an ejector 6 for mixing the hydrogen gas and the hydrogen off-gas, the injector 5 for injection-supplying the hydrogen gas to the ejector 6 and adjusting a flow rate of the hydrogen gas, a plate-shaped first soundproof wall 23 mounting the injector 5 on an upper surface, a soundproof cover 22 mounted on the upper surface of the first soundproof wall 23 so as to cover an outer periphery of the injector 5, and a plate-shaped second soundproof wall 24 mounted by piling up on a lower surface of the first soundproof wall 23. The ejector 6 is arranged between the first soundproof wall 23 and the second soundproof wall 24, and a communication hole 22c for forming a passage to supply a fuel to the injector 5 is formed on the soundproof cover 22.