We constructed a fiber-optic long-term monitoring system for mouse embryo, which developed a new quantitative and non-invasive evaluation method for human embryo. We monitored the changes of light scattering from mouse embryo which might relate to the state of blastomere and change in intercellular microstructures. We employed a couple of optical fibers to measure the scattering. These fibers were arranged in a light angle to the optical axis of a microscope to obtain adaptability for various measurements. A He-Ne laser with 632.8 nm in wavelength was used as a light source. The emission fiber (56 μmΦ; core) and sensing fiber (400 μmΦ; core, NA = 0.43) were used. We observed the light scattering changes from the embryo, which were over 200 times higher than the noise level with 30 s in period. This result might be attributed to the structural and/or functional changes in the mouse embryo.