  Waseda University Nasu telescope array is a spatial fast Fourier transform
(FFT) interferometer consisting of eight linearly aligned antennas with 20 m
spherical dishes. This type of interferometer was developed to survey transient
radio sources with an angular resolution as high as that of a 160 m dish with a
field of view as wide as that of a 20 m dish. We have been performing
drift-scan-mode observations, in which the telescope scans the sky around a
selected declination as the earth rotates. The black hole X-ray binary V404
Cygni underwent a new outburst in 2015 June after a quiescent period of 26
years. Because of the interest in black hole binaries, a considerable amount of
data on this outburst at all wavelengths was accumulated. Using the above
telescope, we had been monitoring V404 Cygni daily from one month before the
X-ray outburst, and two radio flares at 1.4 GHz were detected on June 21.73 and
June 26.71. The flux density and time-scale of each flare were 313+/-30 mJy and
1.50+/-0.49 days, 364+/-30 mJy and 1.70+/-0.16 days, respectively. We have also
confirmed the extreme variation of radio spectra within a short period by
collecting other radio data observed with several radio telescopes. Such
spectral behaviors are considered to reflect the change in the opacity of the
ejected blobs associated with these extreme activities in radio and X-ray. Our
1.4 GHz radio data are expected to be helpful for studying the physics of the
accretion and ejection phenomena around black holes.
