The difference in susceptibility to root-knot nematode, Meloidogyne incognita, between two experimental fields (“Yawara field” and “Katori field”) was studied from 2003 to 2013 using sweetpotato cultivars and lines. The parasitism index to the nematode was similar between the two fields when compared by a mean of 11 years, though the index in each year differed between the two fields in several years. The methods of investigating the parasitism in the two fields slightly differed, but the influence of the method of investigation on the parasitism index seemed to be limited. The nematodes having parasitism similar to the race SP6 was predominant in the Katori field, and they differed from the race SP4 in the Yawara field. It was suggested that several races exist in the Katori field. As it may be difficult to determinate the nematode race only from the parasitism index in field tests, it is necessary to use nematode lines derived from a single egg-mass in order to determinate the nematode race.