The evidence of copper (Cu) ion drift in low-dielectric-constant (low-k) interlayer films is clearly shown by transient capacitance measurements for the first time. The bias-temperaturestress (BTS) was applied to the copper electrode of Cu/low-k/p-Si metal-insulator-semiconductor capacitors. After injecting photoelectrons into the low-k film from the substrate, time-dependence of the capacitance was measured. When BTS-applied samples were measured, a decrement of the capacitance was observed, whereas not observed without BTS. The decrement of the capacitance was attributed to thermal emission of electrons from copper-related electronic states. By assuming an attempt-to-escape frequency of the charge from the electronic states, the energy level of Cu was estimated to be 0.8–0.9 eV below the conduction band edge.