Active Galactic Nuclei (AGNs) often have relativistic jets. Blazars are believed to be viewed at a small angle to the line-of-sight, and show characteristic behaviors for the relativistic effect. Blazars often show highly polarized and variability, because of synchrotron radiation from high energy electrons. Since the synchrotron radiation is polarized perpendicular to a direction of magnetic field, we obtain information of the magnetic field of the emitting region. We performed multi-band optical and near-infrared monitoring to blazars, and then, detected a lot of large-amplitude brightenings, called after outbursts. Most of outbursts occurred with dramatic polarization changes. During the 2010 outburst of 3C 454.3, there were rapid brightenings together with rises of degrees of polarization. This feature indicates that blazar synchrotron radiation is constructed in two components and the outburst component is higher polarized than baseline one.