  We study the correlators for interacting quantum field theory in the flat
chart of de Sitter space at all orders in perturbation. The correlators are
calculated in the in-in formalism which are often applied to the calculations
in the cosmological perturbation. It is shown that these correlators are de
Sitter invariant. They are compared with the correlators calculated based on
the Euclidean field theory. We then find that these two correlators are
identical. This correspondence has been already shown graph by graph but we
give an alternative proof of it by direct calculation.
