  It is pointed out that dipion invariant mass spectrum in $X (3872) \to \pi
\pi J/\psi$ is a useful probe for the $J^{PC}$ quantum number of the X(3872),
complementary to the angular distributions. If X(3872) is a $^1 P_1$ state, the
dipion has a peak at low $m_{\pi\pi}$ region, which is not in accord with the
preliminary Belle data. If X(3872) is a $^3 D_2$ or $^3 D_3$ state, the dipion
spectrum shows a peak at high $m_{\pi\pi}$ region, which is broader than the
$\rho$ resonance that might come from the decay of a molecular state: $(D
\bar{D^*}) (I=1) \to J/\psi \rho \to J/\psi \pi^+ \pi^-$. Better measurement of
$m_{\pi\pi}$ spectrum will shed light on the nature of X(3872).
