  The $2$D Benney-Luke equation is an isotropic model which describes long
water waves of small amplitude in $3$D whereas the KP-II equation is a
unidirectional model for long waves with slow variation in the transverse
direction. In the case where the surface tension is weak or negligible,
linearly stability of small line solitary waves of the $2$D Benney-Luke
equation was proved by Mizumachi and Shimabukuro [Nonlinearity, 30 (2017),
3419--3465]. In this paper, we prove nonlinear stability of the line solitary
waves by adopting the argument by Mizumachi ([Mem. Amer. Math. Soc. no. 1125],
[Proc. Roy. Soc. Edinburgh Sect. A., 148 (2018), 149--198] and
[arXiv:1808.00809]) which prove nonlinear stability of $1$-line solitons for
the KP-II equation.
