The purpose of this research was to see whether music therapy affects positively on some problems of the children with slight mental retardation: concentration problem and maladjusted behavior. Three children with the retardation were chosen as the subjects from an elementary school at Keumsan, Choongnam Province. They were selected for their age similarity and the common characters among them. We selected the children who were similar in age and general characteristics. This research has been operated through sixteen times of individual sessions for eight weeks. To see the changes in slight mentally retarded children\'s concentration and maladjustmentbehavior, AB-design in the research of reverse designed ways was used for the sixteen times of individual sessions. Also, I have done a comparative analysis after the revised edition of Conners teachers (CTRS-28) examination tool\'s marks. \"The time of concentration\"in the sessions was recorded in the way of duration recording and \"the maladjustment behavior\" was recorded in frequency recording. To supplement the weak points from the quantitative data of the standards used in the research, I have analyzed the response about the concentration and maladjustment behavior that had been shown from the children on the music therapy qualitatively. The results of this research are as follows: First of all, this music therapy program improved the concentration time of slight mentally retarded children. Second of all, this music therapy reduced the maladjustment behavior of slight mentally retarded children. Through this research, it implies that music therapy is needed for slight mentally retarded children\'s concentration and maladjustmentbehavior as a rehabilitation program. As a result, it has been confirmed that after music therapy, the concentrationtime of slight mentally retarded children\'s was improved and the maladjustment behavior of the children was reduced. Even though the changes of numerical value are just a few compared to the changes of behaviors has been observed during the sessions, it can be expected better results if the long term program is operated instead of this kind of short period program.