The humanoid robot, WABIAN-2R, has achieved human-like walking with heel contact and toe off motions with a foot mechanism with a passive toe joint. However, the foot structure is different from a human’s. In this paper, we describe a new foot mechanism mimicking the human’s foot arch structure to figure out its function. Especially, the developed foot mimics the elastic properties of the arch and the change of the arch height during walking. We conducted several walking experiments by using WABIAN-2R, and we confirmed that the arch elasticity could absorb a foot-landing impact force at the plantar contact phase and the change of the arch height contributed to a strong thrust at the push-off phase.