In recent years, sweetpotato cultivation is becoming more prevalent in relatively cold regions such as Hokkaido prefecture. We therefore developed a simple low soil temperature testing device to select sweetpotato lines suitable for these relatively cold regions using a ground temperature of 17°C as being suitable for the evaluation and a cultivation period of 3 weeks. Reproducibility of the evaluation was high. To breed high-yield lines suitable for cultivation in relatively cold regions, evaluation using a low soil temperature testing device is insufficient. Subsequent evaluation is necessary of the yield of the selected lines by means of early planting in the field.