The Wheeler-DeWitt equation is investigated and used to examine a state after a quantum tunneling with gravity. To make arguments definite we treat a discretized version of the Wheeler-DeWitt equation and adopt the WKB method. We expand an Euclidean wave function around an instanton, by using a deviation equation of a vector field tangent to a congruence of instantons. The instanton around which we expand the wave function corresponds to a so-called most probable escape path (MPEP). It is shown that, when the wave function is analytically continued, the corresponding state of physical perturbations is equivalent to the vacuum state determined by positive-frequency mode functions which satisfy appropriate boundary conditions. Thus a quantum field theory is effective to investigate a state after a quantum tunneling with gravity. The effective Lagrangian describing the field theory is obtained by simply reducing the original Lagrangian to a subspace spanned by the physical perturbations. The result of this paper does not depend on the operator ordering and can be applied to all physical perturbations, including gravitational perturbations, around a general MPEP. © 1997 The American Physical Society.