  A detailed analysis on the physical conditions of the Interstellar Medium in
the main body of the Local Group dwarf irregular galaxy IC 10 is carried out
using long-slit spectroscopy. Maps of the excitation, the H$\alpha$ flux,
[NII]/H$\alpha$ and [SII]/H$\alpha$ are presented. The Diffuse Ionized Gas
inside the galaxy is studied. We found that the spectral characteristics are
not similar to those for spiral galaxies: the values of the ratios
[NII]/H$\alpha$ and [SII]/H$\alpha$ are not as large as in spiral galaxies but
the excitation is much larger. These values, especially the large excitation,
can be explained by proposing an extra source of ionization. Different sources
are studied but any of the models at hand could be really fitted except a
combination of photons leaking from the HII regions in addition to the
ionization provided by the WR stars present in this galaxy. Shocks are not
needed for explaining the line intensity ratios except in a small part in the
west side of the galaxy.
