The ambient air quality monitoring systems have been recently much developed in many prefectures and cities in Japan. There are huge number of data obtained every day by these systems, which are used as valuable resources, for various kind of administration needs. It is important that the reliability of data should be homogeneous for comprehensive evaluation of data. However the reliability of these data varies depending much upon the extent of data checking. The date checking methods used usually, at present are manual ones in many cases, and not standardized throug hout. Data checking by manual has such problemsas neccecity of enormous volume of work and variety of reliablity of data. Introducing a computerized automatic data checking system may be one of the helpful means for solving them. This paper presents analysis analysis of present state of existing data checking systems, suggestion of several useful methods of off-line data checking system and examination of possibility of adoption of realtime data checking system.