We have developed an in situ system in which material can be inserted into and removed from the plasma apparatus in order to measure the transient variations of plasma with minimum changes in discharge chamber and measurement vessel. By using this system, the transient variations of O-2(+) ion, O+ ion, O radical, electron temperature and electron density were measured when the materials were put into and taken out from the oxygen plasma. Also, the difference between the steady state value without material in the plasma and that with material, in relation to each parameter, was measured. The following facts were found in this study. O-2(+) and O+ mass spectra decreased when the material was inserted into plasma. Then it returned to the original value when the material was taken out. The condition of the transient change of O-2(+) differed remarkably according to the material. By optical emission analysis intensity of O radical was observed more remarkably than that of O-2 ion. Whatever the material was, the transient change did not vary much. The electron density greatly decreased by the material being included into the plasma. However, the electron temperature was constant even if the material was inserted into the plasma. The reflection power was constant by putting material in and out of plasma whatever the material was. The weight decrease in case of Teflon was larger than the other materials after the treatment in the same plasma condition.