A sealing tightness measuring device and a pressure-retaining capability measuring gauge for a pressure container or system is provided. A pressure is produced against the testing fluid inside a piston cylinder communicated with the tested container or system by an assembly including a weight, a piston, and a piston cylinder successively arranged through a coaxial plumb bob. An equation RL=pt/C is used to represent the sealing tightness of the pressure container or system. The equation pt=p(p−0.5Δp)Δt/Δp is used in the pressure-retaining capability measuring gauge to express the pressure-retaining capability, wherein p is the fixed testing fluid pressure, t is the elapsed time for the fluid to leak completely, C is the volume of the pressure container 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.