Measurements of Neutrino Oscillation Angle theta_13

Neutrinos exhibit an interesting phenomenon called 'neutrino oscillation', in which a neutrino changes its flavor after traveling some flight length. Many experiments measured the mixing angles and mass differences, but the angle $\theta_{13}$ had been unmeasured due to its smallness compared to others. During 2011 and 2012, series of new-generation neutrino experiments reported positive results in $\theta_{13}$ search, and its value has been determined to be just below the previous upper limit. The non-zero result of $\theta_{13}$ is a very good news for future of neutrino physics, since it opens a possibility of measuring the CP violation phase in the lepton sector. An introduction to neutrino oscillation and latest experimental results are presented. A detail is put on Double Chooz reactor experiment, in which the author is involved.