Аннотация:MASTER Global robotic net (MASTER-Net:http://observ.pereplet.ru Lipunov et al.,2010, Advances in Astronomy,2010,30L) observed GRB 210402A (Swift trigger 1040123 Page et al. GCN 29743, Evans et al. GCN 29750, Laha et al. 29761, Ttrigger=2021-04-02T22:03:17.22, also Fermi trigger #639093861 T_trigger(Fermi) = 22 04 17 Roberts et al. GCN 29763) at MASTER-IAC, -Tavrida, -SAAO, -Kislovodsk, -OAFA, -Tunka (Lipunov et al. GCN 29741) by MASTER-VWF cameras (two 24.3x16.2sq.deg.) with mlim=13 and by MASTER-II (two 4sq.deg.) with mlims at MASTER cover map https://master.sai.msu.ru/site/master2/observ.php?id=1582249 We observed this field with the MASTER-VWFC of MASTER-IAC, covering the Swift/XRT position from 2021-04-02 22:04:21 to 2021-04-02 22:07:17, i.e. during the prompt emission. Frames were taken with the exposure time of 5s and with no filter . Calibrated with NOMAD V-filter, we marginally see optical source inside T90 at 2 images: started at 2021-04-02 22:05:36(79 from Fermi trigger time) and at 2021-04-02 22:05:46(89 from Fermi trigger) with m_OT\raisebox-0.5ex 11 at XRT position, but the pixel of this camera is 21”. Comparing to the GAIA EDR3, we see no optical counterpart down to the magnitude of 10.5 at other images. It puts the upper limit on the flux of the prompt optical emission at \raisebox-0.5ex 5e-10 erg cm\^-2 s\^-1. Considering that the x-ray flux was \raisebox-0.5ex 1e-08 erg cm\^-2 s\^-1, we constrain the relation between optical flux and x-ray flux at \raisebox-0.5ex 10\^-2.