  The flux of cosmic ray hadrons at the atmospheric depth of 820 g/cm^2 has
been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore,
National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well
described by a single power law : S(E_h) = (2.25 +- 0.21 +- 0.34(sys))
10^(-7)(E_h/1000)^(-2.79 +- 0.05) m^(-2) s^(-1) sr^(-1) GeV^(-1) over the
energy range 30 GeV-30 TeV. The procedure and the accuracy of the measurement
are discussed. The primary proton spectrum is derived from the data by using
the CORSIKA/QGSJET code to compute the local hadron flux as a function of the
primary proton spectrum and to calculate and subtract the heavy nuclei
contribution (basing on direct measurements). Over a wide energy range E_0 =
0.5-50 TeV its best fit is given by a single power law : S(E_0) = (9.8 +- 1.1
+- 1.6(sys)) 10^(-5) (E_0/1000)^(-2.80 +- 0.06) m^(-2) s^(-1) sr^(-1) GeV^(-1).
The validity of the CORSIKA/QGSJET code for such application has been checked
using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the
measured hadron fluxes at the two experimental depths (820 and 1030 g/cm^2
respectively) at better than 10% in the considered energy range.
