  We propose a fixed scale approach to calculate the equation of state (EOS) in
lattice QCD. In this approach, the temperature T is varied by Nt at fixed
lattice spacings. This enables us to reduce T=0 simulations which are required
to provide basic data in finite temperature studies but are quite expensive in
the conventional fixed-Nt approach. Since the conventional integral method to
obtain the pressure is inapplicable at fixed scale, we introduce a new method,
"T-integration method", to calculate pressure non-perturbatively. We test the
fixed scale approach armed with the T-integral method in quenched QCD on
isotropic and anisotropic lattices. Our method is found to be powerful to
obtain reliable results for the equation of state, especially at intermediate
and low temperatures. Reduction of the computational cost of T=0 simulations is
indispensable to study EOS in QCD with dynamical quarks. The status of our
study in Nf=2+1 QCD with improved Wilson quarks is also reported.
