We describe a space-efficient visco-elastic mechanism design for a small lightweight flexible joint of a human-symbiotic robot arm. Human-symbiotic robots that can provide daily life support will be required in houses, offices, facilities, and other places. Since they will often make physical contact with humans, their arms should have mechanical flexible joints to absorb the contact force and follow the human's motion. Our design has two coaxial rotary elements: a torsion bar made from GUMMETAL and a rotary disk damper with the output hole at its center. These two elements are combined in parallel and coaxially, making our design highly space-efficient. This paper presents the specifications of the shear elastic characteristics of GUMMETAL measured in experiments using 1DOF (degree-of-freedom) test equipment. The results confirm that its shear elastic modulus is linear and has a small elastic hysteresis loop and our mechanism can provide the advantage for the robot servo control. Compared with the conventional design, our design can make a smaller lighter flexible joint.