  Motivated by the recent experimental discoveries of multi-quark candidates,
e.g., the $\Theta^+(1540)$, we study multi-quark systems in lattice QCD. First,
we perform accurate mass measurements of low-lying 5Q states with $J=1/2$ and
I=0 in both positive- and negative-parity channels in anisotropic lattice QCD.
The lowest positive-parity 5Q state is found to have a large mass of about
2.24GeV after the chiral extrapolation. To single out the compact 5Q state from
$NK$ scattering states, we develop a new method with the hybrid-boundary
condition (HBC), and find no evidence of the compact 5Q state below 1.75GeV in
the negative-parity channel. Second, we perform the first study of the
multi-quark potential in lattice QCD to clarify the inter-quark interaction in
multi-quark systems. The 5Q potential $V_{\rm 5Q}$ for the QQ-${\rm
\bar{Q}}$-QQ system is found to be well described by the ``OGE Coulomb plus
multi-Y Ansatz": the sum of the one-gluon-exchange (OGE) Coulomb term and the
multi-Y-type linear term based on the flux-tube picture. The 4Q potential
$V_{\rm 4Q}$ for the QQ-${\rm \bar{Q}\bar{Q}}$ system is also described by the
OGE Coulomb plus multi-Y Ansatz, when QQ and $\rm \bar Q \bar Q$ are well
separated. The 4Q system is described as a "two-meson" state with disconnected
flux tubes, when the nearest quark and antiquark pair is spatially close. We
observe a lattice-QCD evidence for the ``flip-flop'', i.e., the flux-tube
recombination between the connected 4Q state and the ``two-meson'' state. On
the confinement mechanism, the lattice QCD results indicate the flux-tube-type
linear confinement in multi-quark hadrons.
