Chapman et al. 2003)\]. They are one of the candidates of forming giant elliptical galaxies based on the current observational and theoretical predictions (Lilly et al. 1999). Although a very limited number of optical counterparts have been identified spectroscopically, they show notable diversity in their morphology (extended and/or merging), AGN possessions, dust temperatures, and so on (Smail et al. 2002a). To study the mechanisms of galaxy formations, such as merging and/or gravitational collapses, we need to clarify the nature and the environments of these faint sub-mm sources. Clearly, we need a much larger sample of faint sub-mm sources than we currently have. It also helps for comparing their features with nearby analogous objects, such as ultra-luminous IRAS galaxies and/or some other dusty populations detected by ISO, MAMBO, etc. These populations might be missed by the popular “Lyman Break galaxy surveys" \[there are only few cases of Lyman Break galaxies with sub-mm detection (e.g. Chapman et al. 2002c)\]. On the other hand, as firstly reported by Cimatti et al.(1998), there is some evidence that many of the sub-mm sources are associated with EROs(extremely red objects)(Cimatti et al. 1998; Blain et al. 2002; Smail et al. 2002a; Ivison et al. 2002; Wehner 