Common epithelial tumors of the ovary, which were found during the preclinical stage in the 5 cases were described, and the histogenesis of ovarian tumors was discussed. 1. All the ovaries in the 5 cases maintained a nearly normal external appearance. As regards the cut surface, 3 cases showed gross-observable tumor mass, and 2 cases had microscopically detectable tumor tissues. All of these tumors were solid, and the cyst formation was not evident to the macroscopical examination. 2. Three cases were serous tumors, and the other 2 cases were of the endometrioid type. Two cases were malignant, and 3 cases were LPM. In two of the cases, the pictures which appeared to correspond to the early stage picture of the unclassified tumor were observed. 3. Non-tumoral inclusion cysts were observed in 4 cases, and the majority of them showed some growth activities. In 2 cases out of these 4, the transient pictures from the active inclusion cyst to the tumor were observed. 4. Tumorgenesis was multicentric based on inclusion cysts, and the tumor proliferated with spreading inside the lymph space in the ovary. The formation of the small cyst and the proliferation in a papillary form were the basic pattern of the tumor.