Abstract: Through the development of the effective field theory of inflation, we can interpret the cosmological perturbation theory as a quantum behavior of the Goldstone boson resulting from the spontaneous breaking of the time translation symmetry. Under this point of view, many wisdoms obtained from the particle physics have been applied to understand the inflationary cosmology. In this talk, we revisit such aspects of the cosmological perturbation theory in the multi-field context, providing more general framework.