  We report the discovery of a cyclotron resonance scattering feature (CRSF) in
the X-ray spectrum of GX 304-1, obtained by RXTE and Suzaku during major
outbursts detected by MAXI in 2010. The peak intensity in August reached 600
mCrab in the 2-20 keV band, which is the highest ever observed from this
source. The RXTE observations on more than twenty occasions and one Suzaku
observation revealed a spectral absorption feature at around 54 keV, which is
the first CRSF detection from this source. The estimated strength of surface
magnetic field, $4.7 \times 10^{12}$ G, is one of the highest among binary
X-ray pulsars from which CRSFs have ever been detected. The RXTE spectra taken
during the August outburst also suggest that the CRSF energy changed over 50-54
keV, possibly in a positive correlation with the X-ray flux. The behavior is
qualitatively similar to that observed from Her X-1 on long time scales, or
from A 0535+26, but different from the negative correlation observed from 4U
0115+63 and X 0331+53.
