To provide a boron amino acid preparation which enables high accumulation of BPA in a cancer, so that when used in a boron-neutron capture therapy the contrast in boron accumulation amounts between the cancer and the surrounding normal tissue by BPA administration can be significantly improved over the prior art, and exposure of normal tissue can be reduced more than in the prior art. A boron amino acid preparation in which 60% or more of L boron amino and D boron amino acid is D boron amino acid, and the boron amino acid is p-boronophenylalanine.