  We study the three nucleon force in Nf=2 dynamical clover fermion lattice
QCD, utilizing the Nambu-Bethe-Salpeter wave function of the three nucleon
system. Since parity-odd two nucleon potentials are not available in lattice
QCD at this moment, we develop a new formulation to extract the genuine three
nucleon force which requires only the information of parity-even two nucleon
potentials. In order to handle the extremely expensive calculation cost, we
consider a specific three-dimensional coordinate configuration for the three
nucleons. We find that the linear setup is advantageous, where nucleons are
aligned linearly with equal spacings. The lattice calculation is performed with
16^3 x 32 configurations at \beta=1.95, m_\pi=1.13 GeV generated by CP-PACS
Collaboration, and the result of the three nucleon force in triton channel is
presented.
