The gamma d to K^+K^-pn reaction has been studied to search for the evidence of the Theta^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 sigma, and the differential cross-section for the gamma n to K^- Theta^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Theta^+ in the gamma n center-of-mass system. The obtained results support the existence of the Theta^+.
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