We directly measured mechanical thermal fluctuations in mirrors over three decades of frequency range using a short-length Fabry-Perot interferometer. This is the first such measurement at wide off-resonant frequency band that is much lower than mechanical resonant frequencies. Theoretically, the mechanical fluctuation in mirrors had been thought to become the principal noise in precise interferometry, such as in gravitational wave detection. We identified the thermally induced noises in the interferometer, the so-called substrate Brownian noise, substrate thermoelastic noise, and coating Brownian noise.