  We report on spectral variability of the blazar 3C 279 in the optical to
X-ray band between MJD 55100 and 58400 during which long-term radio variability
was observed. We construct light curves and band spectra in each of the optical
($2\times10^{14}$-$1.5\times10^{15}$ Hz) and X-ray (0.3-10 keV) bands, measure
the spectral parameters (flux $F$ and spectral index $\alpha$), and investigate
correlation between $F$ and $\alpha$ within and across the bands. We find that
the correlation of the optical properties dramatically change after $\sim$MJD
55500 and the light curves show more frequent activity after $\sim$MJD 57700.
We therefore divide the time interval into three "states" based on the
correlation properties and source activity in the light curves, and analyze
each of the three states separately. We find various correlations between the
spectral parameters in the states and an intriguing 65-day delay of the optical
emission with respect to the X-ray one in state 2 (MJD 55500-57700). We attempt
to explain these findings using a one-zone synchro-Compton emission scenario.
