A fan rotor blade for an aircraft jet engine configured to be fitted in a fitting groove formed in a fan disk, and to take external air into a passage formed in an engine case by rotating integrally with the fan disk, the fan rotor blade comprising: a blade body made from a composite material of a resin and fibers, and including a pressure surface facing one side in a rotation direction and a suction surface facing the other side in the rotation direction; a fitting portion provided as a part of the blade body on one end side of the blade body in a longitudinal direction thereof, the fitting portion being thicker than a remaining region of the blade body and fitted into the fitting groove in the fan disk; and a sheath being more rigid than the blade body, and fixed to the blade body in a state where the sheath covers a leading edge portion of the blade body and the vicinity thereof, the leading edge portion being located on an upstream side in a direction of intake of the external air; wherein the sheath extends in the longitudinal direction of the blade body from the one end side to the other end side of the blade body in the longitudinal direction, and a base end portion of the sheath, which is one end of the sheath in the longitudinal direction, is located closer to the one end side of the blade body than the passage (104, 105) formed in the engine case, and the base end portion of the sheath extends to the fitting portion of the blade body, and covers a leading end of the fitting portion, characterised in that the base end portion of the sheath has a stepped back part on the fitting portion such that it protrudes less than the other portion of the sheath toward the upstream side in the direction of intake of the external air. a blade body made from a composite material of a resin and fibers, and including a pressure surface facing one side in a rotation direction and a suction surface facing the other side in the rotation direction; a fitting portion provided as a part of the blade body on one end side of the blade body in a longitudinal direction thereof, the fitting portion being thicker than a remaining region of the blade body and fitted into the fitting groove in the fan disk; and a sheath being more rigid than the blade body, and fixed to the blade body in a state where the sheath covers a leading edge portion of the blade body and the vicinity thereof, the leading edge portion being located on an upstream side in a direction of intake of the external air; wherein the sheath extends in the longitudinal direction of the blade body from the one end side to the other end side of the blade body in the longitudinal direction, and a base end portion of the sheath, which is one end of the sheath in the longitudinal direction, is located closer to the one end side of the blade body than the passage (104, 105) formed in the engine case, and the base end portion of the sheath extends to the fitting portion of the blade body, and covers a leading end of the fitting portion, characterised in that the base end portion of the sheath has a stepped back part on the fitting portion such that it protrudes less than the other portion of the sheath toward the upstream side in the direction of intake of the external air.