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 _ { h } ( E _ { h } ) = ( 2.25 \pm 0.21 \pm 0.34 ^ { sys } ) \times 10 ^ { -7 } ( \frac { E _ { h } } { 1000 } ) ^ { ( % -2.79 \pm 0.05 ) } m ^ { -2 } s ^ { -1 } sr ^ { -1 } GeV ^ { -1 } over the energy range 30 GeV \div 30 TeV .
The procedure and the accuracy of the measurement are discussed . ( * ) Corresponding author .

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 \div 50 TeV its best fit is given by a single power law : S ( E _ { 0 } ) = ( 9.8 \pm 1.1 \pm 1.6 ^ { sys } ) \times 10 ^ { -5 } ( \frac { E _ { 0 } } { 1000 } ) ^ { ( -2.80 % \pm 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 .