An integrated micro multi-ion sensor was designed and fabricated for a high-performance ion sensor applicable to clinical analyses. Platinum gate ISFETs (ion-sensitive field effect transistors), which have a platinum block layer for protection from ion migration and dissolution and are compatible with MOSFETs, are used. The sensor chip consists of two kinds of ion sensors (K+ and Na+ ISFETs) and two CMOS unity-gain buffers. The output voltages are equal to the equivalent gate membrane potentials of the integrated ISFETs and show a wide linear dynamic range. The K+ and Na+ ISFETs show good chemical responses, including sensitivities, selectivities, and drifts, which are compatible with those of conventional ion-selective electrodes.