Provided is a method for checking defects such as a liquid leakage, a gas leakage, and pack housing damage in a battery pack in which a plurality of unit modules including a battery cell are arranged in a pack housing. The method includes feeding a first gas into the pack housing of the battery pack, in which the first gas has one or more colors selected from red, green, and blue colors in a diffuse state and is visually identifiable in diffusion and flow, checking whether defects occur by visually identifying a point at which a flow direction and a flow rate of the first gas change, when the first gas diffuses in the pack housing; and exhausting the first gas to the outside of the pack housing. The defects of the battery pack are simultaneously identifiable by the first gas diffusing into the entire pack housing.