  We build an ultra-high-speed imaging system based on the background-oriented
schlieren (BOS) technique in order to capture a laser-induced underwater shock
wave. This BOS technique is able to provide two-dimensional density-gradient
field of fluid and requires a simple setup. The imaging system consists of an
ultra-high speed video camera, a laser stroboscope, and a patterned background.
This system takes images every 0.2 $\mu$s. Furthermore, since the density
change of water disturbed by the shock is exceedingly small, the system has
high spatial resolution $\sim$ 10 $\mu$m/pixel. Using this BOS system, we
examine temporal position of a shock wave. The position agrees well with that
measured by conventional shadowgraph, which indicates that the high-speed
imaging system can successfully capture the instantaneous position of the
underwater shock wave that propagates with the speed of about 1500 m/s. The
local density gradient can be determined up to $O$(10$^3$ kg/m$^4$), which is
confirmed by the gradient estimated from the pressure time history measured by
a hydrophone.
