regions of the masses of the Higgs bosons in the $m_{H_3}^{}$ - $m_{H_5}^{}$ plane (a), in the $m_{H_3}^{}$ - $m_{\tilde H_1^0}$ plane (b) and in the $m_{\tilde H_1^0}$ - $m_{\tilde H_1^{0'}}$ plane (c). In Fig. 1(a) the light shadowed regions are excluded by the $Zb\bar b$ results. []{data-label="figure"}](fig1b.eps "fig:"){width="4.9cm"}     ![Allowed regions of the masses of the Higgs bosons in the $m_{H_3}^{}$ - $m_{H_5}^{}$ plane (a), in the $m_{H_3}^{}$ - $m_{\tilde H_1^0}$ plane (b) and in the $m_{\tilde H_1^0}$ - $m_{\tilde H_1^{0'}}$ plane (c). In Fig. 1(a) the light shadowed regions are excluded by the $Zb\bar b$ results. []{data-label="figure"}](fig1c.eps "fig:"){width="4.9cm"}

Fig. 1(a) shows that $m_{H_3}^{}$ and $m_{H_5}^{}$ are bounded from above respectively by about 400 GeV and about 700 GeV. These upper bound come from $|x_1^\pm| < 8\pi$, which give the most stringent constraint among the inequations in Eq.(\[PUconstraint\]). For $m_{H_3}^{} \gsim 170$ GeV, $m_{H_5}$ is bounded from above whose border is approximately corresponding to $\lambda_4 \simeq 0$ or $m_{H_5}^{}\simeq \sqrt{3} m_{H_3}^{}$ with $\theta_H \simeq 0$. Owing to the factor $\sqrt{3}$, the more strict constraint is given on $m_{H_3}^{}$ than on $m_{H_5}^{}$. For $m_{H_3}^{} \lsim 170$ GeV, on the other hand, the upper bound on $m_{H_5}^{}$ (290 GeV) is realized at 