The present invention provides a low-cost hydrogen gas sensor, which exhibits high sensory selectivity for protons and operates at room temperature, and can also provide a highly sensitive sensor capable of fulfilling the important functions of detecting hydrogen gas and preventing leakage accidents in production plants that use hydrogen gas as a carrier, in hydrogen gas storage facilities, and in so-called fuel cells that use hydrogen gas as an energy source. In addition, the sensor is also effective as an acid sensor for hydrofluoric acid and the like.