  We study the three nucleon force in the triton channel using dynamical clover
fermion lattice QCD. The Nambu-Bethe-Salpeter wave function is utilized to
obtain the potentials among three nucleons. Since the straightforward
calculation is prohibitively expensive, two different frameworks are developed
to meet the challenge. In the first method, we study the effective two nucleon
potentials in the three nucleon system, where the differences between the
effective two nucleon potentials and the genuine two nucleon potentials
correspond to the three nucleon system effect, part of which is originated from
the three nucleon force. The calculation is performed using Nf=2 clover fermion
at m(\pi)= 1.13GeV generated by CP-PACS Collaboration, and Nf=2+1 clover
fermion at m(\pi)= 0.70, 0.57GeV generated by PACS-CS Collaboration. In the
second method, we study the three nucleon system with 3D-configuration of
nucleons fixed. This enables us to extract the three nucleon force directly, if
both of parity-even and parity-odd two nucleon potentials are provided. Since
parity-odd two nucleon potentials are not available in lattice QCD at this
moment, we propose a new general procedure to identify the three nucleon force
using only parity-even two nucleon potentials. The calculation are performed
with Nf=2 clover fermion at m(\pi)= 1.13GeV generated by CP-PACS Collaboration,
employing the linear setup for the 3D-configuration. Preliminary results for
the scalar/isoscalar three nucleon force are presented.
