  We discuss a mechanism to generate hierarchy between masses of the top and
bottom quarks without fine-tuning of the Yukawa coupling constants. In the
framework of the two-Higgs-doublet model (THDM) with a discrete $Z_2$ symmetry,
there exists the vacuum where only the top quark receives the mass of the order
of the electroweak symmetry breaking scale $v(\simeq 246 {GeV})$, while the
bottom quark remains massless. We show a model in which a small sofb-breaking
mass $m_3^2 [\sim v^2/(4\pi)^2]$ for the $Z_2$ symmetry is generated by the
dynamics above the cutoff scale of the THDM. The appearance of $m_3^2$ gives
the small mass for the bottom quark, and explain the hierarchy $m_b/m_t \ll 1$.
