We suggest a technique that explicitly accounts for the structure of an
initial state of quantum field in the semiclassical calculations of path
integral in curved space-time, and consider decay of metastable state
(conformal vacuum of scalar particles above false classical vacuum) in
background de Sitter space-time as an example. Making use of this technique, we
justify the Coleman-De Luccia approach to the calculation of the decay
probability. We propose an interpretation of the Hawking-Moss instanton as a
limiting case of constrained instantons. We find that an inverse process of the
transition from true vacuum to false one is allowed in de Sitter space-time,
and calculate the corresponding probability.