For the purpose of analyzing the characteristic of spiral spring, knowing the free shape of spring is indispensable. And to get the free shape of spring needs to analyze on elasticplastic method with geometric non-linearity. But the practical importance for the spring characteristic is the liner zone without both side-ends. If extrapolating liner field to both non-linear zones asuming affine linearity,we are able to analize the spring charactristic only by elasticplastic analysis at both-ends without the necessity of considering geometric nonlinerlity . So we can analyze by easy procedure as compared with traditional integral formulation. This calculation results agree with experimental one in two kind sample springs.