  We present results of near-infrared photometric and spectroscopic
observations of mid-infrared (MIR) sources that dramatically brightened. Using
IRAS, AKARI, and WISE point source catalogs, we found that 4 sources (IRAS
19574+491, V2494 Cyg, IRAS 22343+7501, and V583 Cas) significantly brightened
at MIR wavelengths over the 20-30 years of difference in observing times.
Little is known about these sources except V2494 Cyg, which is considered a FU
Orionis star. Our observation clearly resolves IRAS 22343+7501 into 4 stars
(2MASS J22352345+7517076, 2MASS J22352442+7517037, [RD95] C, and 2MASS
J22352497+7517113) and first JHKs photometric data for all 4 sources are
obtained. Two of these stars (2MASS J22352442+7517037 and 2MASS
J22352497+7517113) are known as T Tau stars. Our spectroscopic observation
reveals that IRAS 19574+9441 is an M-type evolved star and V583 Cas is a carbon
star. 2MASS J22352345+7517076 is probably a YSO, judging from our observation
that it has featureless near-infrared (NIR) spectrum and also showed dramatic
brightening in NIR (about 4 magnitudes in Ks-band). The possible reasons for
dramatic brightening in MIR are discussed in this paper.
