In summer crop regions the storage period of home grown seed tubers is not necessarily short. The yield reduction that is due to planting seed tubers after passing the suitable age and the senile degeneration occur consistently. In each variety, the effect that is due to the advanced age is not so outstanding in the field. However, the decrease in the yield of primes is of different degrees according to the variety. Some indicate almost no decrease, and some nearly 30% decrease. Consequently, in summer crop regions, it is necessary to improve the cultivation by planting seed tubers of a suitable age.