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We present optical (g', R_c, and I_c) to near-infrared (J) simultaneousphotometric observations for a primary transit of GJ3470b, a Uranus-masstransiting planet around a nearby M dwarf, by using the 50-cm MITSuME telescopeand the 188-cm telescope, both at Okayama Astrophysical Observatory. From thesedata, we derive the planetary mass, radius, and density as 14.1 \pm 1.3M_earth, 4.32^{+0.21}_{-0.10} R_earth, and 0.94 \pm 0.12 g cm^{-3},respectively, thus confirming the low density that was reported by Demory etal. based on the Spitzer/IRAC 4.5-micron photometry (0.72^{+0.13}_{-0.12} gcm^{-3}). Although the planetary radius is about 10% smaller than that reportedby Demory et al., this difference does not alter their conclusion that theplanet possesses a hydrogen-rich envelope whose mass is approximately 10% ofthe planetary total mass. On the other hand, we find that the planet-to-starradius ratio (R_p/R_s) in the J band (0.07577^{+0.00072}_{-0.00075}) is smallerthan that in the I_c (0.0802 \pm 0.0013) and 4.5-micron(0.07806^{+0.00052}_{-0.00054}) bands by 5.9% \pm 2.0% and 3.0% \pm 1.2%,respectively. A plausible explanation for the differences is that the planetaryatmospheric opacity varies with wavelength due to absorption and/or scatteringby atmospheric molecules. Although the significance of the observed R_p/R_svariations is low, if confirmed, this fact would suggest that GJ3470b does nothave a thick cloud layer in the atmosphere. This property would offer a wealthof opportunity for future transmission-spectroscopic observations of thisplanet to search for certain molecular features, such as H2O, CH4, and CO,without being prevented by clouds.

Optical-to-Near-Infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint for Cloud-free Atmosphere