Most elderly people complain about the discomfort of movements such as a sit-to-stand (STS) motion through losing their muscular strength of lower extremity over aging. This paper presents a novel passive sit-to-stand and walking (STSW) assistance device to aid in physical support for indoor daily life of the elderly. The STSW assistance device is actuated by a pneumatic cylinder and a gas spring. The standing motion is driven by extension of actuators and the sitting motion is driven naturally with gravitational force of user's weight that reloads actuators. The effect of physical support by using this device was evaluated by experiment with measuring electromyograph signals of subjects during STS motion. As a result, the reduction rate of average maximal voluntary isometric contraction with assistance by the STSW device is about 51.2%. This means that users with the assistive device can stand up by using only about half of muscular activation in the case of natural standing up.