Chemical decontamination is considered to be an effective method for reduction of radiation level by dissolution of radioactive corrosion products and metal oxidizing films existing in the primary system of a nuclear power plant. In this study, the process efficiencies of two chemical decontamination processes (Methods 1 and 2) having different reduction steps were investigated through the operation of a semi-pilot scale decontamination equipment as a continuous work. The reduction step for Method 1 employed an adsorbent with an oxygen source, while a reductant (oxalic acid) was used in the reduction step for Method 2. The dissolution and removal efficiencies of metal species and organic compounds in Method 2 were higher than those in Method 1, implying that oxalic acid in the reduction step increased the process efficiency, their complexes of metal species easily being removed in the decomposition/cleanup step. It was shown that the process employing chemical reduction showed higher dissolution and removal efficiencies rather than the process by the physical adsorption on the adsorbent surface through decontamination processes with different reduction step.