  We investigate relevant deformation and the renormalization group flow in a
defect conformal field theory from the point of view of the holography. We
propose a candidate of g-function in the context of the holography, and prove
the g-theorem: the g-function is monotonically non-increasing along the RG
flow. We apply this g-theorem to the D5-brane solution which is an
asymptotically AdS_4 x S^2 brane in AdS_5 x S^5. This solution corresponds to
the mass deformation of the defect CFT. We checked that the g-function is
monotonically non-increasing in this solution.
