[en] In the fusion devices, only several material probes were employed to study on the wall behavior so far. Then, it was difficult to clarify the plasma wall interactions systematically. In the Large Helical Device (LHD), numerous material probes were installed along the toroidal and poloidal directions from the 1^st to the present 9^th experimental campaigns. After each campaign, the retention-desorption behavior of discharge gases (H,He) and impurity deposition were investigated to understand the plasma wall interactions. The change of the wall surface well corresponded to the plasma discharge behavior and an increase of the plasma stored energy in LHD. After the 3^rd campaign, graphite tiles were installed at the divertor trace regions. The SS wall surface was significantly changed, i.e., largely covered by carbon, so that metal impurity level was reduced. The plasma stored energy very increased by the reduction of metal impurity level. After the 5^th campaign, boronization was three times conducted during each campaign. The oxygen impurity level in the plasma was one or two orders lower, compared to that before the boronization. The toroidal distribution of deposited boron was measured, and it was seen that the wall was partly covered by the boron. These results suggest that the boron very effectively traps the oxygen even if a ratio of the wall coverage is not high. The helium was employed both in main and glow discharges in addition to hydrogen. The helium desorption was often observed during the hydrogen main discharge. The retained amount of helium in the wall was measured to understand this behavior. The amount was comparable to that of hydrogen, which suggests that the helium can easily desorbed from the wall. This helium behavior in the fusion devices has not been known so far. The amounts of retained hydrogen and helium were large in the vicinity of anodes used for the glow discharges. This result shows that the gas retention occurs mainly during the glow discharges. Argon or neon gas was employed for the glow discharge, in order to reduce the amounts of both hydrogen and helium. The low density and high ion temperature discharge was successfully obtained. It was also seen that the amount of retained argon or neon was one order smaller than that of helium. These results suggest that argon or neon glow discharge is useful to reduce particle recycling. (author)