  With a redshift of z=5.5 and an optical blue magnitude M_B ~ -24.2 mag (~4.5
10^12 L_sun), RDJ030117+002025 is the most distant optically faint (M_B > -26
mag) quasar known. MAMBO continuum observations at lambda=1.2 mm (185
micrometer rest-frame) showed that this quasar has a far-IR luminosity
comparable to its optical luminosity. We present near-infrared J- and K-band
photometry obtained with NIRC on the Keck I telescope, tracing the slope of the
rest frame UV spectrum of this quasar. The observed spectral index is close to
the value of alpha_nu ~ -0.44 measured in composite spectra of optically-bright
SDSS quasars. It thus appears that the quasar does not suffer from strong dust
extinction, which further implies that its low rest-frame UV luminosity is due
to an intrinsically-faint AGN. The FIR to optical luminosity ratio is then much
larger than that observed for the more luminous quasars, supporting the
suggestion that the FIR emission is not powered by the AGN but by a massive
starburst.
