  We construct simple and useful approximation for the relativistic gas of
massive particles. The equation of state is given by an elementary function and
admits analytic solution of the Friedmann equation, including more complex
cases when the relativistic gas of massive particles is considered together
with radiation or with dominating cosmological constant. The model of
relativistic gas may be interesting for the description of primordial Universe,
especially as a candidate for the role of a Dark Matter.
