  We report optical spectroscopic observations of four blue-excess
dust-obscured galaxies (BluDOGs) identified by Subaru Hyper Suprime-Cam.
BluDOGs are a sub-class of dust-obscured galaxies (DOGs, defined with the
extremely red color $(i-[22])_{\rm AB} \geq 7.0$; Toba et al. 2015), showing a
significant flux excess in the optical $g$- and $r$-bands over the power-law
fits to the fluxes at the longer wavelengths. Noboriguchi et al. (2019) has
suggested that BluDOGs may correspond to the blowing-out phase involved in a
gas-rich major merger scenario. However the detailed properties of BluDOGs are
not understood because of the lack of spectroscopic information. In this work,
we carry out deep optical spectroscopic observations of four BluDOGs using
Subaru/FOCAS and VLT/FORS2. The obtained spectra show broad emission lines with
extremely large equivalent widths, and a blue wing in the CIV line profile. The
redshifts are between 2.2 and 3.3. The averaged rest-frame equivalent widths of
the CIV lines are $160\pm33$ $\mathrm{\mathring{A}}$, $\sim$7 times higher than
the average of a typical type-1 quasar. The FWHMs of their velocity profiles
are between 1990 and 4470 ${\rm km\ s^{-1}}$, and their asymmetric parameters
are 0.05 and 0.25. Such strong CIV lines significantly affect the broad-band
magnitudes, which is partly the origin of the blue excess seen in the spectral
energy distribution of BluDOGs. Their estimated supermassive black hole masses
are $1.1\times10^8 < M_{\rm BH}/M_\odot < 5.5 \times 10^8$. The inferred
Eddington ratios of the BluDOGs are higher than 1 ($1.1< \lambda_{\rm Edd} <
3.8$), suggesting that the BluDOGs are in a rapidly evolving phase of
supermassive black holes.
