A fuel injection device includes a fuel injection valve at a position corresponding to a portion between intake ports in an internal combustion engine. The fuel injection valve includes a body and a needle inside the body. The fuel injection valve is disposed closer to a piston of the internal combustion engine than the intake ports are to the piston. A single fuel injection passage for conducting gaseous fuel is present between a valve seat of the fuel injection valve and a combustion chamber of the internal combustion engine. The flow cross section of the gaseous fuel in the fuel injection passage is larger on a side closer to the piston of the internal combustion engine than on a side closer to an ignition plug of the internal combustion engine over a specified length in an extending direction of the fuel injection passage.