  Because the timescale of H$\alpha$ emission (several tens of Myr) following
star formation is significantly shorter than that of UV radiation (a few
hundred Myr), the H$\alpha$/UV flux ratio provides insight on the star
formation histories (SFHs) of galaxies on timescales shorter than $\sim100$
Myr. We present H$\alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the
familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive
dataset. The data provide us with robust measurements of dust-corrected SFRs in
both H$\alpha$ and UV for 1,050 galaxies. The results show a correlation
between the H$\alpha$/UV ratio and the deviation from the main sequence in the
sense that galaxies above/below the main sequence tend to have higher/lower
H$\alpha$/UV ratios. This trend increases the dispersion of the main sequence
by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting
that diversity of recent SFHs of galaxies has a direct impact on the observed
main sequence scatter. We caution that the results suffer from incompleteness
and a selection bias which may lead us to miss many sources with high
H$\alpha$/UV ratios, this could further increase the scatter from SFHs in the
star-forming main sequence.
