A study is perfomed on the difference between the volume of gas purchased from Korea Gas Corporation (KGC) and that of measured gas deliveries including total sales, pipeline inventory, and other usage by gas companies at Seoul and Kyunggi area. Actual data md information from 1996 to 2001 on gas purchase from KGC, sales, loss and own company use and pipeline status of each gas company are used to identify net inventory change in pipeline, unaccounted inventory estimates based on billing cycle and their net changes. A framework to be used for the analysis of such differences and an empirical test result are being provided. As a conclusion, the hypothesis that each gas company purchased more than the volume of gas accounted-for by all the possible factors is shown to be rejected.