Radioactive fission gases (krypton and xenon) are observed in boiling water reactor (BWR) plants without defective fuel under various operation conditions. The off-gas in a BWR plant without defective fuel arises from fissile impurities within the cladding materials and/or fissile material deposited on the cladding surface. To estimate the off-gas source in operating plants, the source estimation method considering the off-gas transport time from production to measurement was applied to the data collected under various plant operation conditions. This method was verified by the adaptation of three sets of off-gas data groups to the source of cladding impurity and/or deposition actually observed in the plants. The judgment as to whether off-gas derives from a cladding impurity or a deposition was made by analyzing the data of 11 BWRs with different off-gas levels.