  We present an analytical continuous equation for the Tang and Leschhorn model
[Phys. Rev A {\bf 45}, R8309 (1992)] derived from his microscopic rules using a
regularization procedure. As well in this approach the nonlinear term $(\nabla
h)^2$ arises naturally from the microscopic dynamics even if the continuous
equation is not the Kardar-Parisi-Zhang equation [Phys. Rev. Lett. {\bf 56},
889 (1986)] with quenched noise (QKPZ). Our equation looks like a QKPZ but with
multiplicative quenched and thermal noise. The numerical integration of our
equation reproduce the scaling exponents of the roughness of this directed
percolation depinning model.
