  In this paper, we study transverse linear stability of line solitary waves to
the $2$-dimensional Benney-Luke equation which arises in the study of small
amplitude long water waves in $3$D. In the case where the surface tension is
weak or negligible, we find a curve of resonant continuous eigenvalues near
$0$. Time evolution of these resonant continuous eigenmodes is described by a
$1$D damped wave equation in the transverse variable and it gives a linear
approximation of the local phase shifts of modulating line solitary waves. In
exponentially weighted space whose weight function increases in the direction
of the motion of the line solitary wave, the other part of solutions to the
linearized equation decays exponentially as $t\to\infty$.
