  We study a light dark matter in a radiative neutrino model to explain the
X-ray line signal at about $3.5$ keV recently reported by XMN-Newton X-ray
observatory using data of various galaxy clusters and Andromeda galaxy. The
signal requires very tiny mixing between the dark matter and an active
neutrino; $\sin^2 2\theta\approx 10^{-10}$. It could suggest that such a light
dark matter cannot contribute to the observed neutrino masses if we use the
seesaw mechanism. In other words, neutrino masses might come a structure
different from the dark matter. We propose a model in which Dirac type active
neutrino masses are induced at one-loop level. On the other hand the mixing
between active neutrino and dark matter are generated at two-loop level. As a
result we can explain both the observed neutrino masses and the X-ray line
signal from the dark matter decay with rather mild hierarchy of parameters in
TeV scale.
