  Recently the evidence of the neutrinoless double $\beta$ ($0\nu \beta\beta$)
decay has been announced. This means that neutrinos are Majorana particles and
their mass hierarchy is forced to inverted mass hierarchy (Type B) or
degenerate mass (Type C) patterns in the diagonal basis of charged lepton mass
matrix. We estimate the magnitude of $0\nu \beta\beta$ decay in the degenerate
neutrinos including the cosmological constraint of neutrino dark matter, since
the inverted mass hierarchy pattern always satisfies the cosmological
constraint. The absolute value of neutrino mass is constrained by $0\nu
\beta\beta$ experiment or cosmology depending on the relative signs (Majorana
CP phases) of neutrino masses.
