Japanese magnetospheric explorer GEOTAIL recorded a detailed light curve during the initial 600 msec of a giant flare from SGR 1806-20 on December 27, 2004. We show that the observed light curve is well explained by an emission from relativistically expanding fireballs, like those of gamma-ray bursts (GRBs). Especially, the observed rapid fading after 500 msec suggests that ejecta is collimated in a jet. We derive an upper limit on the jet opening half-angle of 0.2 radian that is as narrow as those of GRBs.