Transient absorption spectroscopy was developed to observe active species produced by light excitation (see figure). Application fields of this technique have been limited because of low sensitivity. Thus, we developed highly sensitive transient absorption spectrometer. In our system, very small absorbance change (< 10^-5) in wide-wavelength-range (400-3000 nm) can be detected with 50 ns time resolution, while 10^-3 absorbance can barely be detected by conventional system. Using the spectrometer, we have studied primary processes of various photo-functional devices, such as dye-sensitized solar cells and photocatalysts. This spectrometer can be also used for trace analysis of molecules in solution.