  Based on the ladder Schwinger-Dyson equation, we investigate phase struc-
ture of the gauged Yukawa model possessing a global $SU(2)_L \times SU(2)_R$
symmetry and an unbroken (vector-like) gauge symmetry. We show that even when
we tune the squared mass of the scalar boson in the Lagrangian to be positive,
there still exists the dynamical chiral symmetry breaking due to fermion pair
condensate (VEV of the composite scalar) triggered by the strong Yukawa
coupling larger than a certain critical value. We find a "nontrivial
ultraviolet fixed line" and "renormalized trajectories" in the
three-dimensional coupling space of the Yukawa coupling, the gauge cou- pling
and the "hopping parameter" of the elementary scalar field. Presence of the
gauge coupling is crucial to existence of the fixed line. Implications of the
result for the lattice calculation and the top quark condensate model are also
discussed.
