This paper proposes an evaluation method of data reliability for environmental monitoring system using wireless sensor networks. The environmental information is often correlated with time and space, and it is effective for environmental monitoring and analysis to form and utilize an approximate expression of envi-ronmental information with the data obtained from a large number of sensors. Therefore, it is also important to establish the evaluation index of the data reliability that can determine how much sensor density and data loss rate are permitted. With the evaluation method, we clarify the correlations between data reliability and some parameters such as orders of the approximation and the number of sensors and data losses.