  We interpret Janus as an interface in a conformal field theory and study its
properties. The Janus is created by an exactly marginal operator and we study
its effect on the interface conformal field theory on the Janus. We do this by
utilizing the AdS/CFT correspondence. We compute the interface free energy both
from leading correction to the Euclidean action in the dual gravity description
and from conformal perturbation theory in the conformal field theory. We find
that the two results agree each other and that the interface free energy scales
precisely as expected from the conformal invariance of the Janus interface.
