ZEUS at electron-proton collider HERA

HERA at DESY in Germany is a single electron-proton collider in the world. Two international collaborations, ZEUS and H1 have been running since the beginning of 1990. Japanese group, participating in ZEUS at its start, has been conducting the inspection of electro-weak interactions, the search for new phenomena beyond the standard model, precise measurements of proton- and photon-structures, and has deepened the understanding of QCD. After the modification of HERA to HERA-II, collision experiments using longitudinally polarized electrons (positrons) are possible, and the inspection of electro-weak interactions is progressing by a means which is complementary to usual particle-antiparticle collisions. (K.Y.)