  We consider an extension of the Standard Model by two right-handed neutrinos,
especially with masses lighter than charged $K$ meson. This simple model can
realize the seesaw mechanism for neutrino masses and also the baryogenesis by
flavor oscillations of right-handed neutrinos. We summarize the constraints on
right-handed neutrinos from direct searches as well as the big bang
nucleosynthesis. It is then found that the possible range for the
quasi-degenerate mass of right-handed neutrinos is $M_N \geq 163 \MeV$ for
normal hierarchy of neutrino masses, while $M_N = 188 \text{--} 269 \MeV$ and
$M_N \geq 285 \MeV$ for inverted hierarchy case. Furthermore, we find in the
latter case that the possible value of the Majorana phase is restricted for
$M_N = 188 \text{--} 350 \MeV$, which leads to the fact that the rate of
neutrinoless double beta decay is also limited.
