New two-A/F systems different from usual A/F-O2 systems are being developed to cope with strict regulation of exhaust gas. In the two-A/F systems, two A/F sensors are equipped in front and rear of a three-way catalyst. The A/F-O2 systems are ideas which use a rear O2 to detect exhaust gas leaked from three-way catalyst early and feed back. On the other hand, the two-A/F systems are ideas which use a rear A/F sensor to detect nearly stoichiometric gas discharged from the three-way catalyst accurately, and to prevent leakage of exhaust gas from the three-way catalyst. Therefore, accurate detection of nearly stoichiometric gas by the rear A/F sensor is the most importrant for the two-A/F systems. In general, the A/F sensors can be classified into two types, so called, one-cell type and two-cell type. Because the one-cell type A/F sensors don’t have hysteresis, they have potential for higher accuracy. In addition, in rear environment, because there are very much condensed water in an exhaust pipe, cup-shaped sensors which are resistant to water splash are suitable. For these reasons, we consider one-cell type A/F sensor for the rear, using a cup-shaped sensor which are resistant to water splash for the first time in the world. We developed the rear-A/F sensor highly accurate for stoichiometric gas successfully, and we report these technologies.