Microelectrode arrays (MEAs) allow us to observe electrical activities from neurons at multiple sites. This paper presents a high-density microelectrode array (HD-MEA) for observing neuronal networks at a cellular level, featuring 236,880 electrodes at an 11.72 μm pitch and 33,840 readout channels with a noise level of 5.5 μVrms. The Peltier cooling system is integrated to maintain the temperature of the electrodes at approximately 37 °C. Moreover, electrical signals for the axonal propagation of rat neurons are successfully recorded.