A tightness-gauging device and a meter for gauging a pressure's sustainability pt for a pressure vessel or system are provided. A pressure is produced against the test fluid inside a piston cylinder communicated with the tested vessel or system by a fixed-pressure supplying assembly including a weight, a piston, and a piston cylinder successively arranged coaxially and vertically. An equation RL=pt/C is used to represent the tightness of the pressure vessel or system. The equation pt=p(p−0.5Δp)Δt/Δp is used in the meter for gauging the pressure's sustainability pt to express the pressure's sustainability, where p is the fixed test pressure, t is the elapsed time for the fluid to leak completely, C is the cubage of the pressure vessel or system, Δp is the drop value of the pressure, and Δt is the elapsed time for the pressure to descend from p to (p−Δp) caused by the leakage.