We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is $Delta m_{15}(B)=0.65pm 0.03$, which is one of the slowest among SNe Ia.
The peak $V$-band absolute magnitude is $M_{V}=-19.90pm 0.15$ mag even if the host extinction is $A_{V}=0$ mag. It reaches $M_{V}=-20.19pm 0.19$ mag for the host extinction of $A_{V}=0.29$ mag as inferred from the observed Na {sc i} D line absorption in the host. Our $JHK_{s}$-band photometry shows that the SN is one of the most luminous SNe Ia also in near-infrared wavelengths. These results indicate that SN 2009dc belongs to the most luminous class of SNe Ia, like SN 2003fg and SN 2006gz. We estimate the ejected $^{56}$Ni mass of $1.2pm 0.3$ $Msun$ for no host extinction case (or 1.6$pm$ 0.4 M$_{odot}$ for the host extinction of $A_{V}=0.29$ mag). The C {sc ii} $lambda$6580 absorption line keeps visible until a week after maximum, which diminished in SN 2006gz before its maximum brightness. The line velocity of Si {sc ii} $lambda$6355 is about 8000 km s$^{-1}$ around the maximum, being considerably slower than that of SN 2006gz, while comparable to that of SN 2003fg. The velocity of the C {sc ii} line is almost comparable to that of the Si {sc ii}. The presence of the carbon line suggests that thick unburned C+O layers remain after the explosion. SN 2009dc is a plausible candidate of the super-Chandrasekhar mass SNe Ia.
