  We report on multi-band photometric and polarimetric observations of the
blazars AO 0235+164 and PKS 1510-089. These two blazars were active in 2008 and
2009, respectively. In these active states, prominent short flares were
observed in both objects, having amplitudes of >1 mag within 10 d. The $V-J$
color became bluer when the objects were brighter in these flares. On the other
hand, the color of PKS 1510-089 exhibited a trend that it became redder when it
was brighter, except for its prominent flare. This redder-when-brighter trend
can be explained by the strong contribution of thermal emission from an
accretion disk. The polarization degree increased at the flares, and reached
>25 % at the maxima. We compare these flares in AO 0235+164 and PKS 1510-089
with other short flares which were detected by our monitoring of 41 blazars.
Those two flares had one of the largest variation amplitudes in both flux and
polarization degree. Furthermore, we found a significant positive correlation
between the amplitudes of the flux and polarization degree in the short flares.
It indicates that the short flares originate from the region where the magnetic
field is aligned.
