A rear arm 15 for a motorcycle obtained by weld-bonding left and right arm molded bodies 2 and 3 made of aluminum alloy die-cast, which are formed by dividing pivot sections 2b and 3b and cross member sections 2c and 3c into left and right parts along a body central line, in the pivot sections and the cross member sections is provided, in which left and the right arm sections 2a and 3a are formed in substantially a triangular shape in side view and formed in substantially C shape opening toward an inner side in a width direction of the vehicle in cross-sectional view, and the cross member sections 2c and 3c are formed in a closed sectional shape extending from vertexes 2g and 3g of the triangular shapes to bases thereof.