The particle trajectories run in the vacuum from the target region to the extension box which is attached to the exit of the D magnet to minimize the multiple scattering effect and/or unnecessary activation of the materials. Figure 18 shows the vacuum chamber of HKS. The vacuum chambers in Q1 and Q2 are shaped to fit the pole faces in order to maximize the solid angle of the spectrometer. The vacuum chamber in the D magnet consists of side walls, and the upper and lower pole faces. They are hermetically sealed with O-rings, and thus there exists a flexibility to adjust the relative position to the Q1, Q2 vacuum chambers. The vacuum extension box is attached to the exit of the D magnet. Thin metal (or Kapton foil reinforced by a Kevler mesh) window, which is placed just in front of the first drift chamber, separates the vacuum system and air. Between the splitter and the Q1 vacuum chamber, a removable sieve slit made of tungsten alloy is placed for the calibration of the HKS spectrometer. The splitter vacuum chamber is also connected to the vacuum chamber of the Enge spectrometer. The photon beam line goes through the Q1 and D magnets to the beam dump. The electron beam deflected by the splitter magnet is guided also through Q1 and D magnets' cuts and is finally bent back to the beam dump by a set of steering magnets. The HKS vacuum chamber is evacuated from bottom of the extension box with a 1000 /min TMP.