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From the viewpoint of global environment and petroleum energy depletion problems, a hydrogen-based fuel cell is attracting people's attentions as a clean energy source. The object of the present paper is to summarize approaches for achieving high stability and high selectivity in developing hydrogen sensors that detect hydrogen gas, fuel of fuel cells. For gas detection principles, various systems including a semiconductor system exist. We are developing hydrogen sensors with thermal conductivity principle applied, in which the difference of gas thermal conductivity is utilized with adaptability in applications for fuel cell electric vehicles taken into account. When the environment in automobile applications is assumed, high selectivity to detect hydrogen alone is required because various gases such as water vapor and exhaust gas (methane, carbon monoxide, etc.) coexist. In particular, the thermal conductivity principle causes big errors at high temperature and high humidity.