  Within the framework of the Gaussian wave-functional approach, we investigate
the influences of quantum and finite-temperature effects on the
Z_2-symmetry(\phi \to -\phi) of the (1+1)-dimensional massive sine-Gordon field
theory. It is explicitly demonstrated that by quantum effects the Z_2-symmetry
can be restored in one region of the parameter space and dynamically
spontaneously broken in another region. Moreover, a finite-temperature effect
can further restore the Z_2-symmetry only.
