The distribution of the local stiffness of a polycrystalline copper exposed to a creep test was studied by resonance ultrasound microscopy. The local effective modulus was evaluated from the resonance frequency of the isolated langasite oscillator touching the specimen. Defects appeared predominantly on grain boundaries, and they were clearly visualized by the stiffness microscopy through the significant decrease of the effective stiffness. The stiffness within the grains becomes lower regardless of invisible defects. The stiffness distribution was quantitatively analyzed by the contact model between two anisotropic bodies and by the micromechanics modeling. The microscopic stiffness shows much higher sensitivity to the defects than the macroscopic stiffness.