Аннотация:MASTER Global robotic net (MASTER-Net:http://observ.pereplet.ru Lipunov et al.,2010, Advances in Astronomy,2010,30L) automatically started (Lipunov et al. GCN 29649) Integral GRB 210312B (Mereghetti et al. GCN 29650, Ttrigger=20:52:17UT) optical observations at MASTER-Kislovodsk at 2021-03-12 20:52:58 UT 27s after notice time(41s after trigger time), and at MASTER-Tunka at 2021-03-12 20:52:58UT with mli18.7 at first 10s images. We confirm Jelinek et al. GCN 29651 (Ondrejov) optical counterpart with unfiltered m_OT=18.2 at first image. This message may be cited.