The linear stability of finite-amplitude surface solitary waves with respect to long-wavelength transverse perturbations is examined by asymptotic analysis for small wavenumbers of perturbations. The sufficient condition for the transverse instability is explicitly derived, and it is found that there exist transversely unstable surface solitary waves whose amplitude-to-depth ratio is greater than 0.713. This critical ratio is well below that for the one-dimensional instability (${=}\,0.781$) obtained by Tanaka (Phys. Fluids, 1986, vol. 29, p. 650).