One of the important breeding issues in buckwheat is to increase resistance to preharvest sprouting, which can result in the loss of yield and quality of buckwheat food, especially in spring-sown buckwheat. The preharvest-sprouting resistance of self-pollinating buckwheat in the summer ecotype is very weak. Therefore, we developed a self-pollinating breeding line IHK1, which has strong preharvest-sprouting-resistance traits. IHK1 was developed by cross breeding between 'Kyukei37' (a preharvest-sprouting-resistant outcrossing line) and IH3 (a self-pollinating summer-ecotype line). Progeny analysis revealed that the preharvest-sprouting resistance trait in IHK1 was dominant. In three field trials conducted over 2 years, the maturation time of IHK1 was the same as that of 'Harunoibuki', the leading spring-sown variety in the Kyushu region and the plant was shorter than 'Harunoibuki'. The yield of IHK1 was approximately 70%-85% of that of 'Harunoibuki' (165 kg/10 a; average of three times cultivation), whereas it was approximately 258%-333% when compared with IH3. Thus, IHK1 is a promising breeding line for self-pollinating buckwheat, especially spring-sown buckwheat.