To provide a gas leakage diagnosis device and a gas leakage diagnosis method that quickly detect the occurrence of gas leakage in a high-pressure tank or the transport piping of gas when the gas leaks, and do not cause any erroneous diagnoses.

The gas leakage diagnosis device 40 diagnoses the gas leakage of a high-pressure tank 11 for storing gas and that of piping 20 for transporting gas. The gas leakage diagnosis device includes: a data acquisition section 51 for acquiring a pressure value P(t) of gas; a differential pressure deriving section 52 for deriving differential pressure P from the pressure value P(t) obtained before and after a setting time T for deriving the differential pressure; a differential pressure verification section 53 for verifying whether the differential pressure P(t) exceeds a setting threshold W by comparing the derived differential pressure P with the setting threshold W; and a determination section 45 for determining that gas has leaked when at least the two diagnoses in different time zones that do not overlap each other in terms of time exceed the setting thresholds W in both.