  We report on the spectral evolution of the enigmatic, very slow nova V5558
Sgr, based on the low-resolution spectra obtained at the Fujii-Bisei
Observatory and the Bisei Astronomical Observatory, Japan during a period of
2007 April 6 to 2008 May 3. V5558 Sgr shows a pre-maximum halt and then several
flare-like rebrightenings, which is similar to another very slow nova V723 Cas.
In our observations, the spectral type of V5558 Sgr evolved from the He/N type
toward the Fe II type during the pre-maximum halt, and then toward the He/N
type again. This course of spectral transition was observed for the first time
in the long history of the nova research. In the rebrightening stage after the
initial brightness maximum, we could identify many emission lines accompanied
by a stronger absorption component of the P-Cygni profile at the brightness
maxima. We found that the velocity of the P-Cygni absorption component measured
from the emission peak decreased at the brightness maxima. Furthermore, we
compared the spectra of V5558 Sgr with V723 Cas, and other novae which
exhibited several rebrightenings during the early phase.
