We have developed an unstable structure (a tumbler with joints) that has circular arc type bottoms as a model of a biped robot walking on rugged terrain. The attitude is controlled by using a small gyroscope. This paper describes the design of the feedback control system which controls the attitude by the support of one leg and the drive of the crotch joints, as the first step prior to a walking experiments for the robot. The basic feedback control system is a one-input two-output system. The H/sub /spl infin// control theory is used to design the compensator. The results of our experiment on the designed control system show that we have successfully stabilized the control of the tumbler system with joints, which is a basic research of biped walking robots.