In neuroscience, it is crucial to clarify the relationship between single-cell activity and neural network structure to understand brain neural activity. To this end, a device that can measure signals with high temporal resolution over a wide area is required. In this study, we developed a multifunctional implantable device for measuring deep-brain functions. The device conducts electrophysiological measurements using microelectrodes and fluorescence imaging using a CMOS image sensor, which enables observations of brain functions with high temporal resolution over a wide area. The device developed is implemented by stacking the microelectrode array and imaging device. We measured the activity of neurons in the ventral tegmental area (VTA) of mice using this device. We successfully recorded action potentials and confirmed that the developed microelectrodes are effective for measuring brain functions. Results suggest that the multifunctional implantable device developed can simultaneously perform electrophysiological measurements and fluorescence imaging using a CMOS image sensor. However, the noise generated during imaging should be eliminated in the future. The activation of γ-aminobutyric acid neurons was also confirmed upon the intraperitoneal injection of nicotine solution as a pharmacological stimulus. The device developed with integrated microelectrodes and a CMOS image sensor is unprecedented and can prove useful in understanding the relationship between neuronal activity and neural networks.