A legged mobile robot leg structure is provided in which a lower leg part (24) is connected to the lower end of an upper leg part (23) through a knee joint (27), a foot part (25) having a six-axis force sensor (60) is connected to the lower end of the lower leg part (24) through an ankle joint (28), and the ankle joint (28) supports the foot part (25) so that the foot part (25) can pitch around a lateral axis (Ly) and roll around a fore-and-aft axis (Lx) relative to the lower leg part (24). A pitch motor (34) for making the foot part (25) pitch around the lateral axis (Ly) is supported on the lower leg part (24) at a position above the ankle joint (28), and a roll motor (35) for making the foot part (25) roll around the fore-and-aft axis (Lx) is supported on the lower leg part (24) at a position above the ankle joint (28). The driving force for driving the lower leg part (24) can thereby be reduced by decreasing the moment of inertia around the knee joint (27), and the influence of noise of the motors (34, 35) on the six-axis force sensor (60) provided on the foot part (25) can be minimized.