  We present the microscopic equation of growing interface with quenched noise
for the Tang and Leschhorn model [{\em Phys. Rev.} {\bf A 45}, R8309 (1992)].
The evolution equations for the mean heigth and the roughness are reached in a
simple way. Also, an equation for the interface activity density (i.e.
interface density of free sites) as function of time is obtained. The
microscopic equation allows us to express these equations in two contributions:
the diffusion and the substratum one. All the equation shows the strong
interplay between both contributions in the dynamics. A macroscopic evolution
equation for the roughness is presented for this model for the critical
pressure $p=0.461$. The dynamical exponent $\beta=0.629$ is analitically
obtained in a simple way. Theoretical results are in excellent agreement with
the Monte Carlo simulation.
