This working paper is a preparatory study to understand how the overall environmental performance of companies can be successfully measured and to introduce how the mechanism for the true revelation of environmental information should be designed by applying policy instrument into the assessment of environmental credibility of companies. The scope to consider the environmental credibility of companies can be of use: a production (or process) side, a product side including product itself and use/disposal of product and other environmental information. To be able to find useful and reasonable insights of measurement indicators within the scope, the next is to develop the structure describing interrelations of categories concerned with measurement criteria. I also consider the mechanism to point out the best policy instrument to press companies to be more transparent. Then, the development of standardized measurement indicators and the algorithm of the rating system for specific industry are followed with the case study in the future study.