Jean Piaget (1896-1980) observed how the intelligence and the knowledge are developed during the span of early childhood and adolescent years, and then he suggested `the stages of cognitive development`. Especially he explained children`s cognitive development with a concept `conservation`. The term conservation refers to those aspects or events that remain constant when other changes are produced in objects or situations. For example, when a ball of clay is rolled into a sausage shape, the form has been altered but the substance, weight, and mass have been conserved. Conservation is obtained when a child learns the function of reversible operation. Some musical educational psychologists, such as M.P. Zimmerman, S.W. Schultz, and G.R. Petzold, made experiments which was intended to apply Piaget`s Theory to musical education. The results of these experimental researches are as follows: 1. The ability to perceive various musical aspects is developed according to the growing of the child. 2. Musical training is the most effective if it is given to children between ages 5 to 7. 3. Conservation of rhythm pattern appears after conservation of tonal pattern has developed. 4. The most meaningful change in the development of auditory cognition takes place between 1st grade and 2nd grade in elementary school. 5. The most sensitive response to the change of the key appears at 3rd grade, and stagnation of conservation of it appears at 4th grade. 6. The response to classify the changes of melody, rhythm, tempo, and mode appears in turn at 5th grade. 7. Instruction on musical structure should depend on the songs which the child has known. 8. Girls achieve better than boys in music at every grade in elementary school. According to these results it is concluded that teachers have to choose active teaching-learing methods which enable children to act and express on the ground of their own cognitive framework, which promote children`s voluntary activity, and which reinforce the relationship between teachers and children. Teachers also need to inquire into more seriously children`s development of musical cognizance such as process of cognizing music, way of musical expression, and musical creativity. This is a way to change a school into a better place for learning music.