Entanglement entropy has recently been recognized to be useful in many purposes, but its general behavior is not well understood because it has been analytically calculated only for limited shapes. To examine its general behavior, we numerically calculated entanglement entropies of an annular region, or its equivalent notion ``mutual information across the annular region'', by space discretization and by the holographic formula.

As a result, we found two interesting features. One is the role of the mutual information as a measure of the UV degrees of freedom in the small width limit. The other is an exponentially small correction to the existing mass expansion formula of the entanglement entropies for gapped theories. Based on these observations, we discuss a possibility that the mutual information can be a measure of degrees of freedom of field theories.