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This paper describes a novel joint mechanism with variable stiffness for a human-friendly robot. In the proposed mechanism, a set of two different hollow-shaped silicone rubber cushions (SRCtrans and SRCstiff : Silicone Rubber Cushion for transmission and for adjusting stiffness, respectively) are sandwiched between an output link and a motor-driven disk. SRCtrans, which is pressed against the motor-driven disk, plays the following three important roles: (1) it transmits rotational motion from the motor to the output link; (2) it absorbs excessive external forces by its elastic deformation and the slippage on the surface of the motor-driven disk; and (3) it allows the estimation of external forces using the information regarding the degree of deformation. On the other hand, SRCstiff, which is placed below SRCtrans, controls the characteristics of joint compliance. SRCtrans is pressed strongly against the motor-driven disk by the expansion of SRCstiff. As a result, the output link can endure the external force without slippage. This paper describes the detail of joint structure and its fundamental performance.