A method of inspecting an internal gas leak in a gas line includes providing an internal gas leak inspection device including a main valve and a pressure gauge in a main line between a first valve and a second valve in the gas line, forming an airtight space in the main line by shielding the main valve and the second valve, measuring a pressure variation of the airtight space using the pressure gauge, and determining whether or not the gas leak occurs, based on the measured pressure variation.