A galvanic cell type of hydrogen sensor was developed using CaZrO2-based proton conductor as a solid electrolyte. It was clear that the hydrogen sensor could be used for the accurate determination of hydrogen concentration in molten aluminum alloys. The sensor probe exhibited stable EMF values and a fast response when the hydrogen concentration in the melt was changed. The application of this sensor to the real aluminum foundry was also investigated. This sensor could work very stably even when the flow rate of the liquid metal was very fast. In situ measurement was possible using this sensor anywhere in the casting shop, such as melting furnace, holding furnace, the degassing station, etc.