We report high precision transit photometry of GJ1214b in JHK _ { s } bands taken simultaneously with the SIRIUS camera on the IRSF 1.4 m telescope at Sutherland , South Africa .
Our MCMC analyses show that the observed planet-to-star radius ratios in JHK _ { s } bands are R _ { p } / R _ { s,J } = 0.11833 \pm 0.00077 , R _ { p } / R _ { s,H } = 0.11522 \pm 0.00079 , R _ { p } / R _ { s,Ks } = 0.11459 \pm 0.00099 , respectively .
The radius ratios are well consistent with the previous studies by \citet 2011ApJ…743…92B within 1 \sigma , while our result in K _ { s } band is shallower than and inconsistent at 4 \sigma level with the previous measurements in the same band by \citet 2011ApJ…736…78C .
We have no good explanation for this discrepancy at this point .
Our overall results support a flat transmission spectrum in the observed bands , which can be explained by a water-dominated atmosphere or an atmosphere with extensive high-altitude clouds or haze .
To solve the discrepancy of the radius ratios and to discriminate a definitive atmosphere model for GJ1214b in the future , further transit observations around K _ { s } band would be especially important .