  We report the recent progress on the determination of three-nucleon forces
(3NF) in lattice QCD. We utilize the Nambu-Bethe-Salpeter (NBS) wave function
to define the potential in quantum field theory, and extract two-nucleon forces
(2NF) and 3NF on equal footing. The enormous computational cost for calculating
multi-baryon correlators on the lattice is drastically reduced by developing a
novel contraction algorithm (the unified contraction algorithm). Quantum
numbers of the three-nucleon (3N) system are chosen to be (I, J^P)=(1/2,1/2^+)
(the triton channel), and we extract 3NF in which three nucleons are aligned
linearly with an equal spacing. Lattice QCD simulations are performed using
N_f=2 dynamical clover fermion configurations at the lattice spacing of a =
0.156 fm on a 16^3 x 32 lattice with a large quark mass corresponding to
m(\pi)= 1.13 GeV. Repulsive 3NF is found at short distance.
