  We address the infrared(IR) divergence problem during inflation that appears
in the loop corrections to the primordial perturbations. In our previous paper,
we claimed that, at least in single field models, the IR divergence is
originating from the gauge artifact. Namely, diverging IR corrections should
not appear in genuine gauge-invariant observables. We propose here one simple
but explicit example of such gauge-invariant quantities. Then, we explicitly
calculate such a quantity to find that the IR divergence is absent at the
leading order in the slow-roll approximation for the usual scale invariant
vacuum state. At the same time we notice that there is a subtle issue on the
gauge-invariance in how to specify the initial vacuum state.
