We developed a new hydrogen sensor using Mg-Ni switchable mirror thin film. It can be operated without heating with a wide detection range. Also combination use of a switchable mirror thin film and slab wave guide enables to realize a hydrogen sensor with high sensitivity. Using a switchable mirror thin film coated sheet, we can visualize hydrogen diffusion process directly, which is a new way of hydrogen sensing by means of our eyes. This hydrogen visualization sheet will be commercialized shortly.