  We followed-up the massive young stellar object (MYSO) S255-NIRS3
(=S255-IRS1b) during its recent accretion outburst event in the Ks band with
Kanata/HONIR for four years after its burst and obtained a long-term light
curve. This is the most complete NIR light-curve of the S255-NIRS3 burst event
that has ever been presented. The light curve showed a steep increase reaching
a peak flux that was 3.4 mag brighter than the quiescent phase and then a
relatively moderate year-scale fading until the last observation, similar to
that of the accretion burst events such as EXors found in lower-mass young
stellar objects. The behavior of the Ks band light curve is similar to that
observed in 6.7 GHz class II methanol maser emission, with a sudden increase
followed by moderate year-scale fading. However, the maser emission peaks
appear 30-50 days earlier than that of the Ks band emission. The similarities
confirmed that the origins of the maser emission and the Ks band continuum
emission is common as previously shown from another infrared and radio
observations by Stecklum et al. (2016); Caratti o Garatti et al. (2017a);
Moscadelli et al. (2017). However, the differences in energy transfer paths,
such as the exciting/emitting/scattering structures, may cause the delay in the
flux-peak dates.
