  The lowest scalar and pseudoscalar glueball masses are evaluated by means of
the time-dependent variational approach to the Yang-Mills gauge theory without
fermions in the Hamiltonian formalism within a Gaussian wavefunctional
approximation. The glueball mass is calculated as a pole of the propagator for
a composite glueball field which consists of two massless gluons. The glueball
propagator is here evaluated by using the linear response theory for the
composite external glueball field. As a result, a finite glueball mass is
obtained through the interaction between two massless gluons, in which the
glueball mass depends on the QCD coupling constant g in the nonperturbative
form.
