In this study, we analyze the factors that influence the naturalness of Japanese alveolar stops as well as the realizations of the length when Korean learners attempt to produce it. Using different data models with both visual and auditory stimulation, we conducted experiments on reading paradigm tasks as well as delayed repetitive tasks. Japanese native speakers judged the naturalness of the length of the Korean learner's voice obtained from both tasks. Statistical analysis was performed based on the judgements of the results, and factors affecting the realization of the length were analyzed. As a result, it was clarified that the Japanese alveolar stops were particularly arranged in the words. The type and length of the following vowels had the strongest influence on the naturalness of the mora length. In particular, the unnatural rate of words followed by the short vowel /e/ and the long vowel /ee/ was significantly higher. If the Japanese voiced plosive [d] on the second mora is followed by the short vowel /e/, it turned out that the unnatural rate was also higher. The learning level of the subject was the strongest factor in the difference in the generation method between the two tasks. In both tasks, the unnatural rate of beginners was high, but the factor that affected the unnatural rate was the position of the plosive in the word.