  We investigate how an equation of state for matter is affected when a gravity
is present. For this purpose, we consider a box of ideal gas in the presence of
Newtonian gravity. In addition to the ordinary thermodynamic quantities, a
characteristic variable that represents a weight per unit area relative to the
average pressure is required in order to describe a macroscopic state of the
gas. Although the density and the pressure are not uniform due to the presence
of gravity, the ideal gas law itself is satisfied for the thermodynamic
quantities when averaged over the system. Assuming that the system follows an
adiabatic process further, we obtain a {\it new} relation between the averaged
pressure and density, which differs from the conventional equation of state for
the ideal gas in the absence of gravity. Applying our results to a small volume
in a Newtonian star, however, we find that the conventional one is reliable for
most astrophysical situations when the characteristic scale is small. On the
other hand, gravity effects become significant near the surface of a Newtonian
star.
