  We estimate the Big Bang nucleosynthesis (BBN) constraint on the majoron in
the mass range between $1\,{\rm MeV}$ to $10\,{\rm GeV}$ which dominantly
decays into the standard model neutrinos. When the majoron lifetime is shorter
than $1\,{\rm sec}$, the injected neutrinos mainly heat up background plasma,
which alters the relation between photon temperature and background neutrino
temperature. For a lifetime longer than $1\,{\rm sec}$, most of the injected
neutrinos directly contribute to the protons-to-neutrons conversion. In both
cases, deuterium and helium abundances are enhanced, while the constraint from
the deuterium is stronger than that from the helium. $^7{\rm Li}$ abundance
gets decreased as a consequence of additional neutrons, but the parameter range
that fits the observed $^7{\rm Li}$ abundance is excluded by the deuterium
constraint. We also estimate other cosmological constraints and compare them
with the BBN bound.
