  A naive computation of the correlation functions of fluctuations generated
during inflation suffers from logarithmic divergences in the infrared (IR)
limit. In this paper, we propose one way to solve this IR divergence problem in
the single-field inflation model. The key observation is that the variables
that are commonly used in describing fluctuations are influenced by what we
cannot observe. Introducing a new perturbation variable which mimics what we
actually observe, we propose a new prescription to solve the time evolution of
perturbation in which this leakage of information from the unobservable region
of the universe is shut off. We give a proof that IR divergences are absent as
long as we follow this new scheme. We also show that the secular growth of the
amplitude of perturbation is also suppressed, at least, unless very higher
order perturbation is discussed.
