To provide a container having a degassing mechanism which can be reliably sealed during the storage, can smoothly emit gas outside from the container when the gas pressure in the container is increased, and can prevent the container from breaking.

A piston is provided on a middle plug assembled to a mouth part of a container body, the piston is actuated according to the pressure in the container, and an air hole communicated with the outside of the container is opened/closed thereby. More specifically, the middle plug assembled to the container body comprises an outer cylindrical body having the outside diameter substantially equal to the inside diameter of the mouth part, a middle plug body having an inner bottomed cylindrical body continuously formed on the inner side of the outer cylindrical body, a piston slidably assembled to the inner cylindrical body, a spring member for urging the piston, and an air hole formed in a bottom plate of the inner cylindrical body. In an inner peripheral surface of the inner cylindrical body, a lower half from the bottom plate to the predetermined height is formed into a smooth surface tightly attached to the outer periphery of the piston, and an upper half is formed into an uneven surface having recessed bars or projecting bars extending in the longitudinal direction.