  We calculate the $B \to D^{(*)} \ell \nu$ form factors in 2+1 flavor
relativistic lattice QCD by employing the M\"obius domain-wall action for all
quark flavors. Our simulations are carried out at lattice cut-offs $a^{-1} \sim
2.5$, 3.6 and 4.5 GeV with the bottom quark masses up to 0.7 $a^{-1}$ to
control discretization effects. We extrapolate the form factors to the
continuum limit and physical quark masses, and discuss systematic uncertainties
of the form factors.
