  We present a new pulsar wind nebula (PWN) model solving both advection and
diffusion of non-thermal particles in a self-consistent way to satisfy the
momentum and energy conservation laws. Assuming spherically symmetric (1--D)
steady outflow, we calculate the emission spectrum integrating over the entire
nebula and the radial profile of the surface brightness. We find that the back
reaction of the particle diffusion modifies the flow profile. The photon
spectrum and the surface brightness profile are different from the model
calculations without the back reaction of the particle diffusion. Our model is
applied to the two well-studied PWNe 3C 58 and G21.5-0.9. By fitting the
spectra of these PWNe, we determine the parameter sets and calculate the radial
profiles of X-ray surface brightness. For both the objects, obtained profiles
of X-ray surface brightness and photon index are well consistent with
observations. Our model suggests that particles escaped from the nebula
significantly contribute to the gamma-ray flux. A $\gamma$-ray halo larger than
the radio nebula is predicted in our model.
