To date, many control methods have been studied, assuming that the soles of a biped walking robot contact the ground as four points. It is difficult for a biped robot with rigid and flat soles to maintain four-point contact on uneven terrain. It means that the biped robot can lose its balance. To solve this kind of problem, we should study not only stability control methods but also foot mechanisms. In this paper, a new foot system, WS-1 (Waseda Shoes - No.1), is proposed to maintain four-point contact. This foot system consists of cam-type locking mechanism. WS-1 is attached to the feet of WL-16 (Waseda Leg - No.16) that is the world's first biped-walking robot capable of carrying a human. Through hardware experiments, the effectiveness of the foot system is confirmed.