The present studies reported in this paper were made in an effort to make clear the basic facts of dry matter production in sweet potato under field conditions. Sweet potato, variety Norin No. 1, was grown on a feild at 1961 with different fertility levels, planting dates and planting densities. Dry matter increasing was measured at intervals of 3 weeks over all growing course. Maximum rate of dry matter increase per unit field area showed 120 g/m2 field/week which existed at middle stage of growth. Maximum values of net assimilation rate and leaf area index were 55 g/m2 leaf area/week and 4.3 m2/m2 field, respectively. Optimum leaf area index of sweet potato on field for the dry matter production was 3.2 on solar radiation 380 g.cal/cm2/day at middle growing stage, from July 17 to Sept. 18. When leaf area index became all excess, it had an unfavourable effect on the dry matter production due to lack of light intensity in community, moreover, stems weight/leaves weight ratio tend to increase. Increasing of this ratio was considered to be disadvantage for the net assimilation rate owing to rising the respiratory consumption in non-productive parts. Table 1. Total dry weight and tuberous rots weight at harvesting time. [table] Note. 1). Harvesting time…Early planting: Oct. 10. Late planting: Oct. 31. 2). Abbreviation……P: poor soil. F: fertile soil. D: dense planting. T: thin planting. 3). T is same as in Fig. 9.