Mn-55 nuclear magnetic resonance (NMR) measurements have been carried out in Mn12 clusters in order to investigate microscopically the magnetic properties of inner spin structure of the molecule in its high spin S=10 ground state. The external magnetic field dependence of Mn-55-NMR spectrum gives a direct confirmation of the internal magnetic structure of the Mn12 cluster, in which spin moments of Mn4+ (S=3/2) ions are polarized antiparallel to that of Mn3+ (S=2) ions. It is proved that the microscopic spin configuration rotates rigidly when a field up to 8T is applied perpendicular to the easy-axis.