This research proposes a methodology for flow path research named In-Activity Measuremen(IAM), aiming to build a systematic method which makes an in-detph and clear exploration of flow path . IAM is different from common flow experience measurements such as Narrative Survey, Active Survey, and Experience Sampling Method (ESM), in that it can measure the flow experience of the subject who will not be distracted by answering questions.Futhermore, IAM provides the possibility of doing research on massive subjects by analyzing data and forming correspondences. In order to demonstrate the feasibility and the practical usage of IAM, Learning Technology Lab develops a casual rhythm game called Music Flow, so as to collect real data and to analyze them for the purpose of explaining every procedure of IAM, moreover, to explore the flow path of casual game players. The research procedures of IAM are the followings:(1)Set up the evaluation index(2)Obtain the sample data(3)Analyze the sample data(4)Form the correspondences (5)Obtain the regular data(6) Analyze the regular data(7)Analyze the flow path It’s worth noting that IAM can be used not only for gaming activities. Any activity involving flow experience can be studied with it if researchers are able to build the data collecting platform and to conform the research procedures of IAM.