  FASER is one of the promising experiments which search for long-lived
particles beyond the Standard Model. In this paper, we focus on dark photon
associating with an additional U(1) gauge symmetry, and also a scalar boson
breaking this U(1) gauge symmetry. We study the sensitivity to the dark photon
originated from U(1)-breaking scalar decays. We find that a sizable number of
dark photon signatures can be expected in wider parameter space than previous
studies.
