The main aim of this article is to show some intimate relations among the following three notions: (1) the metaplectic representation of
Sp(2n,R) and its extension to some semigroups, called the Olshanski semigroup for
Sp(2n,R) or Howe's oscillator semigroup, (2) antinormally-ordered quantizations on the phase space
R2m≅Cm, (3) path integral quantizations where the paths are on the phase space
R2m≅Cm. In the Main Theorem, the metaplectic representation
ρ(eX) (
X∈sp(2n,R)) is expressed in terms of generalized Feynman--Kac(--Itô) formulas, but in real-time (not imaginary-time) path integral form. Olshanski semigroups play the leading role in the proof of it.