  In order to understand the structure of the X(3872), the $c \bar c$
charmonium core state coupling to the $D^0 \bar D^{\ast 0}$ and $D^+ D^{\ast
-}$ molecular states are studied. The strengths of the couplings between the
charmonium state and the hadronic molecular states are determined so as to
reproduce the observed mass of the the X(3872). The isospin symmetry breaking
is introduced by the mass differences of the neutral and charged $D$ mesons.
The obtained structure of the X(3872) is that about 7% is $\ccbar$ charmonium,
75% is the isoscalar $D \bar D^\ast$ molecule and 18% is the isovector $D \bar
D^\ast$ molecule, which explains observed properties of the X(3872) well. The
structure of the X(3872) we have obtained is not just a $D^0 \bar D^{ast 0}$
hadronic molecule nor the compact tetraquark state but the charmonium-hadronic
molecule hybrid state.
