Thus, it is needed to search for ERO counterparts to those sub-mm sources that are optically bright enough for detailed spectroscopic studies.

Using the gravitational lensing effect as a natural telescope, which amplifies the brightness of distant objects behind the lens, is one of the most powerful methods for obtaining detailed information on intrinsically faint objects. There have been many successful results in observing various types of high redshift galaxies using this effect, such as Lyman break galaxies(Pettini et al. 2000; Franx et al. 1997), faint SCUBA sources(Ivison et al. 2000; Ivison et al. 2001; Ledlow et al. 2002) and EROs(G.P. Smith et al. 2002). The advantages of using the lensing effect is not only its photometric, but also spatial magnification, which will provide enlarged images of smaller distant objects, corresponding to a very high effective spatial resolution, which could not be achieved by the usual ground based observations. By using these effects, we could investigate the detailed structures and also the environmental status of distant objects.

In order to search for bright counterparts of SCUBA sources and to investigate their surrounding environments in detail, we performed ERO search observations behind 2 massive clusters of galaxies, MS $0451.6-0305$ ($z = 