A new hydrogen sensing system for molten metals usable up to 1500 K was developed by using two sort of galvanic cells employing proton conducting oxide CaZr0.9In0.1O3-delta and stabilized zirconia Zr0.91Mg0.09O2-delta, respectively as the electrolytes. The system is founded on the principle that the accurate value of hydrogen potential at the measuring electrode of the galvanic cell based on CaZr0.9In0.1O3-delta can be obtained theoretically from its emf and the value of oxygen potential at the same electrode which is determined by an auxiliary galvanic cell based on Zr0.91Mg0.09O2-delta. The validity of the system was confirmed by experiments in a closed chamber using a controlled atmosphere. The performance of the system was checked in the practical melting processes of several industrial plants. These results showed that the system was very useful to monitor the hydrogen potentials in the plants.