A fuel injector includes a body having a fuel injection port, and a seal ring on the body. The body has a diameter-shrink portion of which an outer diameter is shrunk. The seal ring is engaged with the diameter-shrink portion. The diameter-shrink portion has a radially concaved annular groove with which a tip end portion of the seal ring is engaged. It is restricted that the seal ring is rubbed against an inner surface of the assembling hole and the seal ring is damaged.