  WASP-80b is a warm Jupiter transiting a bright late-K/early-M dwarf,
providing a good opportunity to extend the atmospheric study of hot Jupiters
toward the lower temperature regime. We report multi-band, multi-epoch transit
observations of WASP-80b by using three ground-based telescopes covering from
optical (g', Rc, and Ic bands) to near-infrared (NIR; J, H, and Ks bands)
wavelengths. We observe 5 primary transits, each of which in 3 or 4 different
bands simultaneously, obtaining 17 independent transit light curves. Combining
them with results from previous works, we find that the observed transmission
spectrum is largely consistent with both a solar abundance and thick cloud
atmospheric models at 1.7$\sigma$ discrepancy level. On the other hand, we find
a marginal spectral rise in optical region compared to the NIR region at
2.9$\sigma$ level, which possibly indicates the existence of haze in the
atmosphere. We simulate theoretical transmission spectra for a solar abundance
but hazy atmosphere, finding that a model with equilibrium temperature of 600 K
can explain the observed data well, having a discrepancy level of 1.0$\sigma$.
We also search for transit timing variations, but find no timing excess larger
than 50 s from a linear ephemeris. In addition, we conduct 43 day long
photometric monitoring of the host star in the optical bands, finding no
significant variation in the stellar brightness. Combined with the fact that no
spot-crossing event is observed in the five transits, our results confirm
previous findings that the host star appears quiet for spot activities, despite
the indications of strong chromospheric activities.
