This paper describes an innovative passive element for assistive device and its application. S-FRP is an elastic composite consisting of FRP and rubber. S-FRP provides restoring force in 3 dimension. A prototype of S-FRP and its basic characteristics are shown. Moreover, a prototype of KEIROKA-assistive suit using S-FRP for supporting back bend motion is developed. The authors show experimental results that the prototype suit can reduce 20% of muscular activations in back bending and stretching motions.