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 N(t) 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-N(t) 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 N(f) = 2 + 1 QCD with improved Wilson quarks is also reported.