We apply the effective field theory approach to
quasi-single field inflation, which contains an additional scalar field with
Hubble scale mass other than inflaton.
Based on the time-dependent spatial diffeomorphism, which is not broken
by the time-dependent background evolution, the most generic action of
quasi-single field inflation is constructed up to third order
fluctuations. Using the obtained action,
the effects of the additional massive scalar field on the primordial curvature
perturbations are discussed. In
particular, we calculate the power spectrum and discuss the momentum-dependence
of three point functions in the squeezed limit for general settings of
quasi-single field inflation. Our
framework can be also applied to inflation models with heavy particles. We make
a qualitative discussion on the effects of heavy particles during inflation and
that of sharp turning trajectory in our framework.