In order to see where the background comes from, the beam line from the exit of main linear accelerator(linac) to the interaction point(IP) is shown in Fig.2 for JLC-I(500GeV).  A bending magnet of +20mrad, which is the so-called big bend, is needed because the background from the upstream linac can not directly hit a detector. The second bending magnet of -16mrad makes a horizontal crossing of mrad between two beams at IP. There are two major sections of 1200m collimation and a 600m final focus system in the beam line, where (1)muons and (2)synchrotron radiation are produced, respectively. At IP, (3) pairs and (4)minijets are created in beam-beam interactions. In the subsequent sections these four kinds of background are briefly discussed together with the possible designs of an interaction region against them.
Figure 2: Top view of the beam line from the exit of the main linac to the interaction point (IP) of JLC-I(500GeV).