A fuel injection valve mounting structure in which: a fuel injection part located at a front end of a fuel injection valve is fitted into a mounting hole of an engine, the fuel injection valve including a coupler on an intermediate portion thereof in an axial direction; a fuel supply cap provided in a retaining member is fitted onto an outer periphery of a fuel inlet part located at a rear end of the fuel injection valve, the retaining member being fixed to the engine; and positioning means for restricting an orientation of the coupler in a given direction is provided between the retaining member and the fuel injection valve, wherein a covering body made of a synthetic resin and being integral with the coupler is formed around an outer periphery of an intermediate portion in the axial direction of the fuel injection valve, the covering body including a rear shoulder part oriented toward the fuel inlet part side, characterized in that the rear shoulder part includes: a first semicircular shoulder part extending over a half periphery of the covering body; a second semicircular shoulder part shifted in the axial direction with respect to the first semicircular shoulder part and extending over the other half periphery of the covering body; and a standing wall standing to connect the first and second semicircular shoulder parts (16a, 16b), and a semicircular holding surface and a rotation stopper surface are formed on a front end of the fuel supply cap, the holding surface holding at least one of the first and second semicircular shoulder parts (16a, 16b) to prevent the fuel injection valve from being fallen off from the mounting hole, the rotation stopper surface standing from the holding surface to abut against the standing wall and thereby forming the positioning means, wherein the second semicircular shoulder part is disposed frontward of the first semicircular shoulder part, and the holding surface holds the second semicircular shoulder part. a fuel injection part located at a front end of a fuel injection valve is fitted into a mounting hole of an engine, the fuel injection valve including a coupler on an intermediate portion thereof in an axial direction; a fuel supply cap provided in a retaining member is fitted onto an outer periphery of a fuel inlet part located at a rear end of the fuel injection valve, the retaining member being fixed to the engine; and positioning means for restricting an orientation of the coupler in a given direction is provided between the retaining member and the fuel injection valve, wherein a covering body made of a synthetic resin and being integral with the coupler is formed around an outer periphery of an intermediate portion in the axial direction of the fuel injection valve, the covering body including a rear shoulder part oriented toward the fuel inlet part side, characterized in that the rear shoulder part includes: a first semicircular shoulder part extending over a half periphery of the covering body; a second semicircular shoulder part shifted in the axial direction with respect to the first semicircular shoulder part and extending over the other half periphery of the covering body; and a standing wall standing to connect the first and second semicircular shoulder parts (16a, 16b), and a semicircular holding surface and a rotation stopper surface are formed on a front end of the fuel supply cap, the holding surface holding at least one of the first and second semicircular shoulder parts (16a, 16b) to prevent the fuel injection valve from being fallen off from the mounting hole, the rotation stopper surface standing from the holding surface to abut against the standing wall and thereby forming the positioning means, wherein the second semicircular shoulder part is disposed frontward of the first semicircular shoulder part, and the holding surface holds the second semicircular shoulder part.