  We propose a model to explain tiny masses of neutrinos with the lepton number
conservation, where neither too heavy particles beyond the TeV-scale nor tiny
coupling constants are required. Assignments of conserving lepton numbers to
new fields result in an unbroken $Z_2$ symmetry that stabilizes the dark matter
candidate (the lightest $Z_2$-odd particle). In this model, $Z_2$-odd particles
play an important role to generate the mass of neutrinos. The scalar dark
matter in our model can satisfy constraints on the dark matter abundance and
those from direct searches. It is also shown that the strong first-order phase
transition, which is required for the electroweak baryogenesis, can be realized
in our model. In addition, the scalar potential can in principle contain
CP-violating phases, which can also be utilized for the baryogenesis.
Therefore, three problems in the standard model, namely absence of neutrino
masses, the dark matter candidate, and the mechanism to generate baryon
asymmetry of the Universe, may be simultaneously resolved at the TeV-scale.
Phenomenology of this model is also discussed briefly.
