The term ‘visualization' has been used in various fields of society in recent years. ‘Visualization' was originally used in fluidics and then in natural sciences, social sciences and human sciences. In the fields of evaluation and decision aids, ‘visualization' is required for accountability, transparency and freedom in information systems. However, it seems the idea of ‘visualization' is still indigested. In this paper, first we searched for articles that mention ‘visualization' and then outline past research on ‘visualization'. Next, we discuss ‘visualization', especially focusing on decision aids. Specifically, we organize the structure and problems of ‘visualization', first the aims and the positions for ‘visualization', second the stage of ‘visualization', and third the contents and techniques of ‘visualization'. Consequently, the following results were obtained: 1) it is necessary to carefully consider the gaps between the subject and object for ‘visualization', 2) ‘visualization' must fit the decision aid process, 3) it is necessary to choose appropriate methods for the particular ‘visualization' purpose. Finally, we should not forget that ‘visualization' may create ‘un-visualization' as lighting makes shadows. JFL classification: D70, D81, D82, Q51, D53