A cross member is arranged so as to extend in a width direction of a vehicle body. The cross member has both ends fixed to side framework structures of the vehicle body. The cross member includes a base frame extending in the width direction and reinforcing frame parts formed in integral with the base frame to cover the circumference of the base frame within its limited ranges in the width direction of the vehicle body. The base frame and the reinforcing frame parts are made from the same resinous material or plural resinous materials belonging to engineering plastic system.