  In this paper we present and discuss our time-resolved photometry of an
eclipsing recurrent nova, U Sco, during an outburst in 1999, which was
conducted from immediately after the optical maximum to the final fading toward
the quiescence. In the first-ever complete light-curve, a few primary and
secondary eclipses of the binary system were detected, and the timings of the
minima were determined. We found that the eclipses showed no totality during
the outburst. The depth of the primary eclipses was 0.4-0.8 mag, much shallower
than that in quiescence. In the plateau phase, very little irradiation (< 0.1
mag) was observed in the orbital light curve, which implies the existence and a
large flaring rim of the accretion disk during the outburst. The minima of the
eclipses were detected at earlier orbital phases for the predicted ephemerides.
Thus, we obtained an orbital period change of the binary system as \dot{P}/P =
-1.7 (+/- 0.7) x 10^{-6} yr^{-1} from the O-C. Assuming that this period change
is a result of the conservative mass transfer between the component stars, its
mass-transfer rate reaches \dot{M} = 2.4 (+/- 1.0) x 10^{-6} M_solar yr^{-1}
for a 1.37 M_solar white dwarf and a 2.0 M_solar mass-donor companion, which is
too high to cause shell flashes, even on a massive white dwarf. Therefore, this
large rate of the period change strongly indicates a non-conservative mass
transfer in the binary system.
