  We propose one way to regularize the fluctuations generated during inflation,
whose infrared (IR) corrections diverge logarithmically. In the case of a
single field inflation model, recently, we proposed one solution to the IR
divergence problem. There, we introduced new perturbative variables which
better mimic actual observable fluctuations, and proved the regularity of
correlation functions with respect to these variables. In this paper, we extend
our previous discussions to a multi field inflation model. We show that, as
long as we consider the case that the non-linear interaction acts for a finite
duration, observable fluctuations are free from IR divergences in the multi
field model, too. In contrast to the single field model, to discuss
observables, we need to take into account the effects of quantum decoherence
which pick up a unique history of the universe from various possibilities
contained in initial quantum state set naturally in the early stage of the
universe.
