  We investigate the relation between the star-formation activity in galaxies
and environment at z=1.2 in the COSMOS field, using the fraction of [OII]
emitters and the local galaxy density. The fraction of [OII] emitters appears
to be almost constant over the surface density of galaxies between 0.2 and 10
Mpc^-2. This trend is different from that seen in the local universe where the
star-formation activity is weaker in higher density regions. To understand this
difference between z~1 and z~0, we study the fraction of non-isolated galaxies
as a function of local galaxy density. We find that the fraction of
non-isolated galaxies increases with increasing density. Our results suggest
that the star formation in galaxies at z~1 is triggered by galaxy interaction
and/or mergers.
