Sweetpotato is among the most important crops originating from the tropics. Low soil temperature inhibits the thickening of sweetpotato roots. The growth of ordinary sweetpotato cultivars is thought to be limited below 15 - 16℃. In Japan, commercial sweetpotato production has recently been attempted in cool areas, in addition to warm areas. And a device for testing low-temperature tolerance in sweetpotato has been developed. In this system, containers fabricated from aluminum boards were filled with horticultural soil and set in a cooled water tank. Sweetpotatoes were cultivated at a soil temperature of 17℃ for three weeks. Some lines exhibiting slight growth in low soil temperature conditions (14℃) were successfully bred, and achieved high yield from early planting in fields. It is important to breed low-temperature-tolerant sweetpotato lines achieving both high yield and high quality, in order to expand the crop’s cultivation range.