  We present the results of our lattice QCD study of the $g_{D^* D\pi}i$
coupling, relevant to the $D^\ast \to D\pi$ decay. Our computation is made on
the gauge field configurations that include $N_f=2$ dynamical light quarks by
using the twisted mass QCD action. From the results obtained at four different
lattice spacings we were able to take for the first time the continuum limit of
this quantity computed on the lattice. Our final value, $g_c=0.53(3)(3)$, leads
to $\Gamma(D^{\ast +}\to \bar{D}^0 \pi^+)= (50\pm 5\pm 6)$ keV, and is in good
agreement with the experimental results for the width of the charged
$D^\ast$-meson.
