(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.