The big wave of digital fabrication movement has spread to several educational areas, such as those related to STEAM education. However, there are not so many researches to evaluate instructional design for digital fabrication education. Therefore, we conducted an experiment in an actual digital fabrication course and evaluated its instructional design. The research question of the experiment is what kind of factors affect to knowledge retention of learners in the case of digital fabrication education. In the experiment, learners took a test to assess knowledge they acquired in lessons each week. They were required to repeatedly try to take the tests until they achieved a perfect score. Three weeks later, to assess learner's knowledge retention, the learners took a delayed-test which was the same contents as the previous tests. Several learners scored lower than a half of full marks even though they achieved the perfect score three weeks before. In order to investigate the reason why they could not retain their knowledge, we analyzed their job-sheet they had written their works and found several reasons of them caused from lack of their practical experience. In consideration of the analytical result, this paper also discusses about a better instructional design for digital fabrication. The contents of the discussion will contribute as a useful idea for an instructor to consider instructional design for digital fabrication education.