Tremendous progress has been achieved in neutrino oscillation physics during the last few years. However, the smallness of the $t13$ neutrino mixing angle still remains enigmatic. The current best constraint comes from the CHOOZ reactor neutrino experiment $s2t13 < 0.2$ (at 90% C.L., for $adm2=2.0 10^{-3} text{eV}^2$). We propose a new experiment on the same site, Double-CHOOZ, to explore the range of $s2t13$ from 0.2 to 0.03, within three years of data taking. The improvement of the CHOOZ result requires an increase in the statistics, a reduction of the systematic error below one percent, and a careful control of the cosmic ray induced background. Therefore, Double-CHOOZ will use two identical detectors, one at $sim$150 m and another at 1.05 km distance from the nuclear cores. The plan is to start data taking with two detectors in 2008, and to reach a sensitivity of 0.05 in 2009, and 0.03 in 2011.
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