The purpose of this study is to develop concept map as a tool of performance assessment in mathematics and to conduct feasibility test of the task. I developed and applied a concept map, an assessment tool, in the form of a mathematics performance task for 1st and 2nd year middle school students. I also conducted feasibility test of the task, and at the same time sought ways to guarantee reliability of the assessment procedure. Results of the task assessment can be summarized into the following four points. First, the concept map task was found to be appropriate for an assessment of mathematics in middle school. Second, results of all 10 factors on verification of validity of the content of the task were positive with over 50%. Third reliability of teachers who conducted marking of the four concept map tasks was high, all recording over .871 in correlation number.