We examine the `singlet majoron model' first introduced by Chikashige,
Mohapatra and Peccei as a simple extension of the standard model with massive
Majorana neutrinos. We can explain both the solar and the atmospheric neutrino
deficits by the oscillations between electroweak doublet and singlet neutrinos
without flavor mixing. Furthermore, while some light neutrinos can be the hot
dark matter, tau neutrino with mass of 8.9-24 MeV can be the cold dark matter
through the interaction with the majoron. Thus, we can simultaneously explain
the solar neutrino deficit, the atmospheric neutrino anomaly, and the cold and
hot dark matters only with the Majorana neutrinos.Comment: 14 pages, uses REVTEX macr
