Pharmaceutical contaminants such as diclofenac (DCF) cannot be removed in existing wastewater treatment facilities; therefore, studies on application of new treatment processes and improvement of efficiency are required. In this study, a modified photocatalyst doped with nitrogen and europium was prepared and the performance of DCF was evaluated. A modified photocatalyst that responds to visible light was prepared by precipitating nitrogen and europium in a TiO2 powder using a plasma-in-liquid process (PLP). The performance of the photocatalyst was evaluated by a degradation experiment of diclofenac, a pharmaceutical ingredient. The dopant tended to precipitate in proportion to the amount of precursor added, but more nitrogen precipitated than europium even when the same amount was added. Nitrogen and europium were dispersed evenly throughout the TiO2 powder, and the Ti2p peak position of the modified TiO2 photocatalyst (MTP) coincided with bare TiO2, and europium precipitated in the form of Eu2O3. The bandgap energy of the MTPs was lower than that of unmodified TiO2 photocatalyst, but the MTP with only europium precipitated was the lowest. When a blue light source in the visible region was used, DCF decomposition by MTPs was improved by about 15 to 25 times compared to bare TiO2, and europium precipitation photocatalyst had the highest DCF decomposition characteristic. In addition, MTPs showed excellent reusability properties. Four kinds of by-products were detected in the decomposition process of DCF, and three decomposition pathways by reactions such as decarboxylation, C-N cleavage and hydroxylation were considered. The final mineralization to H2O, CO2, and chlorine occurs by hydroxylation, such as by OH, on the MTP.