This paper proposes a walking assist machine for the elderly or rehabilitants, which uses a spatial parallel link mechanism. Its flat steps move in parallel with the ground and can support the whole legs of a person including his/her soles. A prototype was developed, which was designed to assist the halfpower of walking. Its performance was evaluated by experimental load tests corresponding to the half weight of each leg. Furthermore, to confirm the validity of our supporting method, muscle activity of the leg while wearing the walking assist machine was evaluated by using the surface electromyography (EMG) and calculating the ratio of the integrated EMG (IEMG) with and without the walking assist machine. The experimental results show that the activities of the rectus femoris muscle and the tibialis anterior muscle were particularly decreased to approximately sixty percent by wearing the walking assist machine.