  We study the possibility that the dilaton -- the fundamental scalar field
which exists in all the existing unified field theories -- plays the role of
the dark matter of the universe. We find that the condition for the dilaton to
be the dark matter strongly restricts its mass to be around 0.5 keV or 270 MeV.
For the other mass ranges, the dilaton either undercloses or overcloses the
universe. The 0.5 keV dilaton has the free-streaming distance of about 1.4 Mpc
and becomes an excellent candidate of a warm dark matter, while the 270 MeV one
has the free-streaming distance of about 7.4 pc and becomes a cold dark matter.
We discuss the possible ways to detect the dilaton experimentally
