In this study, we have investigate the concept of the 5th and 6th grade elementary school students on the pressure of the gas using the network analysis. The instrument was developed for estimating students`s understanding of the concepts relating to the origin, direction and some properties of the gas. It consist of 7 items including 2 multiple choice items and 5 two-tier mutiple choice items. We can summarize our results as follows: 1. A large portion of the 5th and 6th grade students appeared to have understood the origin of the pressure of the gas in terms of the weight of the air (5th:83.2%, 6th:58.3%), and a relatively large portion of the 6th grade students also had understood it in terms of the molecular motion comparing with the 5th grade students (5th:26.5%, 6th:41.7%). 2. Both the 5th and 6th grade students appeared to have correct concept on the direction of the pressure of the gas (5th:67.3%, 6th:72.5%), but, relatively large portion of the 5th grade students showed not to have understood it such that the pressure of the gas acted only on the outside of the inflated ball. 3. The 5th grade students seemed to explain the properties of the gas more intuitively than the 6th grade students, who explained the same phenomena with more scientific ideas.