To judge the understanding level of students unfamiliar with programming on a sample code, we propose a specification change exercise. In a specification change exercise, a teacher indicates a tiny specification change on the sample code. He forces students to modify the given sample code to meet the new specification. To automate marking, the method conducts a specification change exercise, using a control structure graph. A control structure graph stratifies elements constructing the sample code in the order of abstraction level. Each node in the graph has one-on-one relation with a code fragment in the sample code. When students identify the code fragments to be modified, they select its nodes instead of code fragments. The comparison of nodes selected by students with the correct nodes designated by teachers enables the automatic calculation of the understanding level on the sample code. Using calculated level, the method provides students study time, and facilitates students to enhance understanding level gradually by themselves. As a result of experiment, the method greatly reduces human load to judge the understanding level assuring almost same understanding level as existing human based way.