Massive stars play an important role in the galaxies and cosmic evolution. However,
detailed mass-growth processes of massive young stellar objects (MYSOs) were not clear
until now. Because the formation of massive stars is deeply obscured by dense gas and
dust, and is located relatively far from us, their physical structures are hard to
resolve.

Variability study is a promising method for this purpose. Such studies of low-mass young
stellar objects at optical and infrared wavelengths have proven to be powerful tools for
deciphering the physics of star formation and pre-main-sequence stellar
evolution. However, few works were made in MYSO studies. Here we show our recent works
of 1) MYSOs' variability search using WISE archival data and 2) follow-up NIR
monitoring of the accretion outburst MYSO S255-NIRS3.

In the first work, we found 5 variable MYSOs and they show various time and color flux
variation. It indicates various origins of variability, such as change in accretion rate
or line-of-sight extinction. We also found a periodic variable MYSO (~ 700 days),
possibly binary interaction origin.

In the second work, we obtained the most complete NIR light-curve of S255-NIRS3 and
showed its characteristics are similar to EXors, lower-mass accretion burst YSOs. We
also discussed relation between class II methanol maser light-curves and origin of
observed time-delay.