A fan rotor blade constructed of a composite material composed of thermosetting or thermosoftening plastic and reinforced fiber to be utilized in a fan for introducing air into an engine flow passage formed in an inside of an engine case of an aircraft engine, the fan rotor blade comprising: a blade body that is constructed of the composite material and has a suction-side surface on one side thereof and a pressure-side surface on another side thereof; a blade root integrally formed at a base end of the blade body by the composite material and capable of being joined with a joint slot formed on an outer circumference of a fan disc of the fan; and a metallic sheath attached to a leading edge of the blade body so as to extend in a span direction of the fan rotor blade for protecting the leading edge, wherein the sheath includes a sheath main body that gradually decreases in thickness toward a leading edge of the fan rotor blade and a pair of joint flanges (55, 57) integrally extended from both rear edges of the sheath main body, the rotor blade being characterised in that: the sheath is segmented into a sheath base segment on a base end side of the blade body and a sheath top segment on a top end side of the blade body, and the sheath top segment smoothly continues from the sheath base segment, and is not longer than the sheath base segment along the span direction, and further characterized in that: a sheath length "k" of the sheath main body at a position of 80% span from a base end of the sheath satisfies: 10% chord ≤ k ≤ 60% chord, and a sheath length "m" of the sheath along an end edge of the fan rotor blade satisfies: m ≥ 40% chord. a blade body that is constructed of the composite material and has a suction-side surface on one side thereof and a pressure-side surface on another side thereof; a blade root integrally formed at a base end of the blade body by the composite material and capable of being joined with a joint slot formed on an outer circumference of a fan disc of the fan; and a metallic sheath attached to a leading edge of the blade body so as to extend in a span direction of the fan rotor blade for protecting the leading edge, wherein the sheath includes a sheath main body that gradually decreases in thickness toward a leading edge of the fan rotor blade and a pair of joint flanges (55, 57) integrally extended from both rear edges of the sheath main body, the rotor blade being characterised in that: the sheath is segmented into a sheath base segment on a base end side of the blade body and a sheath top segment on a top end side of the blade body, and the sheath top segment smoothly continues from the sheath base segment, and is not longer than the sheath base segment along the span direction, and further characterized in that: a sheath length "k" of the sheath main body at a position of 80% span from a base end of the sheath satisfies: 10% chord ≤ k ≤ 60% chord, and a sheath length "m" of the sheath along an end edge of the fan rotor blade satisfies: m ≥ 40% chord. the sheath is segmented into a sheath base segment on a base end side of the blade body and a sheath top segment on a top end side of the blade body, and the sheath top segment smoothly continues from the sheath base segment, and is not longer than the sheath base segment along the span direction, and further characterized in that: a sheath length "k" of the sheath main body at a position of 80% span from a base end of the sheath satisfies: 10% chord ≤ k ≤ 60% chord, and a sheath length "m" of the sheath along an end edge of the fan rotor blade satisfies: m ≥ 40% chord. a sheath length "k" of the sheath main body at a position of 80% span from a base end of the sheath satisfies: 10% chord ≤ k ≤ 60% chord, and a sheath length "m" of the sheath along an end edge of the fan rotor blade satisfies: m ≥ 40% chord.