  We derive a simple calculation rule for Aoyama--Tamra's prescription for path
integral with degenerated potential minima. Non-perturbative corrections due to
the restricted functional space (fundamental region) can systematically be
computed with this rule. It becomes manifest that the prescription might give
Borel summable series for finite temperature (or volume) system with quantum
tunneling, while the advantage is lost at zero temperature (or infinite volume)
limit.
