To obtain an assembly by putting a beverage based on a kind of milk containing a specified amount of nitrous oxide or the like into a container having a specific volume of an upper space saturated with nitrogen so as to have a specific relative pressure to contain the ready-to-drink beverage of long life under a pressurized state.

This assembly is obtained by putting a beverage based on a kind of milk pref. such as diluted milk, common milk, condensed milk, coffee flavor imparting agent-added milk or flavor imparting agent-added milk containing 0.5-5 g/L of nitrous oxide or carbon dioxide into a container (pref. a metallic packaging material, bottle or the like) having an upper space accounting for 9-15 vol.% of the total volume of the container and saturated with nitrogen so as to have a relative pressure of 1.5-4 bar to contain the ready-to- drink beverage of long life under a pressurized state.