  Spin-physics projects at J-PARC are explained by including future
possibilities. J-PARC is the most-intense hadron-beam facility in the
high-energy region above multi-GeV, and spin physics will be investigated by
using secondary beams of kaons, pions, neutrinos, muons, and antiproton as well
as the primary-beam proton. In particle physics, spin topics are on muon $g-2$,
muon and neutron electric dipole moments, and time-reversal violation
experiment in a kaon decay. Here, we focus more on hadron-spin physics as for
future projects. For example, generalized parton distributions (GPDs) could be
investigated by using pion and proton beams, whereas they are studied by the
virtual Compton scattering at lepton facilities. The GPDs are key quantities
for determining the three-dimensional picture of hadrons and for finding the
origin of the nucleon spin including partonic orbital-angular-momentum
contributions. In addition, polarized parton distributions and various hadron
spin topics should be possible by using the high-momentum beamline. The
strangeness contribution to the nucleon spin could be also investigated in
principle with the neutrino beam with a near detector facility.
