In recent years metacognition has been receiving increased attention in mathematics education. Special attention has been focused on metacognition and its essential role in achievement settings. The basic difficulty of the study on the field of metacognition is to develop and use valid tasks in order to measure metacognitive ability especially for young pupils. The present study is a part of a larger research on the development of young pupils' metacognitive ability in mathematics. It represents the initial phase of an instrument development and the examination of its construct validity. The confirmatory factor analysis confirms the existence of a second order structure representing metacognition and two basic first order factors indicating metacognitive knowledge and metacognitive regulation. (Contains 1 figure and 1 table.) [For complete proceedings, see ED500858.]