1.0cm [**PACS index : 12.60.Fr, 14.80.Cp** ]{} 1.0cm [**Keywords : Non-standard model, Partial-wave unitarity, Higgs boson mass bounds**]{}

Introduction
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The nature of electroweak symmetry breaking remains unknown at the present status of our knowledge for high energy physics. In the standard model (SM), a scalar isospin doublet field, the Higgs field, is introduced to be responsible for spontaneous breakdown of electroweak gauge symmetry. Its vacuum expectation value (VEV) triggers the symmetry breaking, so that it provides origins of masses of weak bosons via the Higgs mechanism, and also does those of quarks and charged leptons via Yukawa interaction. Although the SM Higgs sector is simple, the Higgs sector could have a more complicated structure in the actual world. In particular, when the Higgs sector would play an additional role to explain phenomena which the SM cannot, it should necessarily be an extended form from the SM one. Therefore, experimental detection of the Higgs boson and precision measurements of its properties are extremely important not only to confirm our basic idea of electroweak symmetry breaking but also to determine details of the Higgs sector and further to outline the structure of new physics.

In constructing an extended Higgs sector, there 