  We explore three-nucleon forces (3NF) from lattice QCD simulations. Utilizing
the Nambu-Bethe-Salpeter (NBS) wave function, two-nucleon forces (2NF) and 3NF
are determined on the same footing. Quantum numbers of the three-nucleon (3N)
system are chosen to be (I, J^P)=(1/2,1/2^+) (the triton channel). The enormous
computational cost is reduced by employing the simplest geometrical
configuration, where 3N are aligned linearly with an equal spacing. We perform
lattice QCD simulations using Nf=2 dynamical clover fermion configurations
generated by CP-PACS Collaboration, 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.
