A rear vehicle body structure includes left and right auxiliary members each connected at the front end to the underside of the respective left or right front floor frame and connected at the rear end to the respective left or right rear-suspension mounting section. Each of the left and right auxiliary members is generally in the form of a circular-section pipe and has a pipe section of a circular sectional shape and a flattened section formed on a front end portion of the pipe section. The flattened section has an upper surface portion flattened downwardly and has a lower surface portion having a central region flattened concavely upwardly along the axis of the auxiliary member.