To detect readily and precisely increase of pressure inside a container up to a specific pressure, by constituting a detector, when the pressure inside the container increases up to a specific pressure by pressurizing, so that a detection piston is pressed out to the outside of a detector main body against an elastic force of an elastic body.

When a pressure P in a fuel tank 1 ascends up to a value near a predetermined maximum height by pumping of a hand operation pressurizing mechanism, a detection piston 23 is moved to an external direction of the fuel tank 1 against a compression coil spring 26, and a white flange 24a of a display body 24 provided in a tip portion of the detection piston 23 is exposed from a hole part 21b of a detector main body 21 to outside. Therefore, it is recognized that a pressure in the fuel tank 1 approaches the maximum. When the pressure in the fuel tank 1 ascends further and reaches the maximum, as the detection piston 23 is further moved to outside against a spring 26 and a red display part 25 is exposed to outside, it is recognized that the pressure in the fuel tank 1 reaches the maximum value, for example 2.5MPa.