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In most of the Nambu·Jona-Lasinio(NJL)-type models, realizing the hidden chiral symmetry, the existence of a scalar particle σ is needed with a mass mσ = 2mq, as a partner of the Nambu-Goldstone boson π. However, the results of many analyses on ππ phase-shift thus far made have been negative for its existence.In this paper we re-analyze the phase-shift, applying a new method, the interfering amplitude method, which treats the T -matrix directly and describes multi-resonances in conformity with the unitarity. As a result, the existence of σ has been strongly suggested from the behavior of the ππ → ππ phase shift between the ππ-and the KK-thresholds, with mass = 553.3 ± 0.5stMeV and width= 242.6 ± 1.2stMeV. The most crucial point in our analysis is the introduction of a negative background phase, possibly reflecting a "repulsive core" in ππ interactions.The properties of f0(980) are also investigated from data including those over the KK threshold. Its mass is obtained as 993.2 ± 6.5st ± 6.9sysMeV. Its width is about a hundred MeV, although this depends largely on the treatment of the elasticity and the ππ → KK phase shift, both of which may have large experimental uncertainties. * ) In the ENJL model is predicted the existence of NG boson, scalar meson, and moreover, vector and axial-vector meson nonets. typeset using PTPT E X.sty * * ) However, see the analyses 9), 10), 11) which suggest the existence of σ. * ) See, e.g., Refs. 12) and 13). * * ) Owing to Watson's final state interaction theorem, the ππ production amplitude of all types of reactions is closely related to the amplitude of ππ scattering. Accordingly, if this concentration is actually due to the σ-particle production, the corresponding phase shift ought to be observed in ππ scattering.

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