A concept of learning efficiency, the degree of achievement increased per unit time, was presented to determine the optimum pacing. To examine the learning efficiency, the learning tasks were chosen in the text gooks which were Science II of the middle school. The degree of achievement grew higher with the teaching time. But the learning efficiency was the highest at the first test and it was gradually decreased after the first class. When the test items were classified into the knowledge, the comprehension and the application level by Bloom`s taxonomy, about a half of the items belonged to the knowledge level. In addition tow that, with the separate analysis of items of the comprehension and the application, it was showed that the learning efficiency was the highest at the second test. It was thought that the optimum learning time be lengthened because the more complex cognitive structure was required to solve the test items of the comprehension and the application than those of knowledge. When the optimum learning time determined in this research was compared with the current learning time practiced in the secondary school, the practical learning time was relatively appropriate for memorizing the knowledge but insufficient for understanding and applying it.