A diagnosis device diagnoses current cutoff devices connected in parallel and disposed on an energization path to an energy storage device mounted on a vehicle. The diagnosis device performs switch processing of switching one of the current cutoff devices to be diagnosed from an opened state to a closed state or from the closed state to the opened state and closing the other current cutoff device while an engine of the vehicle is stopped. The diagnosis device detects end-to-end voltage of the current cutoff device when current larger than a threshold flows through the current cutoff device after the switch processing, and diagnoses the current cutoff device based on the detected end-to-end voltage.