A blade (37, 17) for a fan in an aircraft engine, comprising: a blade body (39, 19) having a ventral surface on one side in a blade thickness direction, and a back surface on the other side in the blade thickness direction; and a platform (41, 21) formed at a base end of the blade body (39, 19), including a flow passage surface of air outside in a radial direction, a first flange is formed on an upstream end side in an opposite surface of the flow passage surface of the platform (41, 21), a second flange is formed on a downstream end side in the opposite surface of the flow passage surface of the platform(41, 21), characterised in that a first rib for reinforcing the platform (41, 21) is formed in at least either of a portion of the ventral surface side and a portion of the back surface side in the opposite surface of the flow passage surface of the platform (41, 21), the first rib extends toward the upstream end of the platform (41, 21) from a first position separated to an upstream side by 8 to 30% of an inter-flange distance between the first flange and the second flange with respect to an axial center of the platform (41, 21), a second rib for reinforcing the platform (41, 21) is formed in the at least either portion in the opposite surface of the flow passage surface of the platform (41, 21), the second rib extends toward the downstream end of the platform (41, 21) from a second position separated to a downstream side by 8 to 30% of the inter-flange distance with respect to the axial center of the platform (41, 21), the first rib does not reach the first flange, and the second rib does not reach the second flange. a blade body (39, 19) having a ventral surface on one side in a blade thickness direction, and a back surface on the other side in the blade thickness direction; and a platform (41, 21) formed at a base end of the blade body (39, 19), including a flow passage surface of air outside in a radial direction, a first flange is formed on an upstream end side in an opposite surface of the flow passage surface of the platform (41, 21), a second flange is formed on a downstream end side in the opposite surface of the flow passage surface of the platform(41, 21), characterised in that a first rib for reinforcing the platform (41, 21) is formed in at least either of a portion of the ventral surface side and a portion of the back surface side in the opposite surface of the flow passage surface of the platform (41, 21), the first rib extends toward the upstream end of the platform (41, 21) from a first position separated to an upstream side by 8 to 30% of an inter-flange distance between the first flange and the second flange with respect to an axial center of the platform (41, 21), a second rib for reinforcing the platform (41, 21) is formed in the at least either portion in the opposite surface of the flow passage surface of the platform (41, 21), the second rib extends toward the downstream end of the platform (41, 21) from a second position separated to a downstream side by 8 to 30% of the inter-flange distance with respect to the axial center of the platform (41, 21), the first rib does not reach the first flange, and the second rib does not reach the second flange.