To provide a production method capable of forming a container continuously producing a damper enclosing viscous liquid, in a container made by closing a flexible film in a bag shape, at an efficient productivity.

In production of a film damper 16 in which a viscous liquid is enclosed in a container 20 formed by closing a flexible film 18 in a bag shape. While a pair of long flexible films 18 are fed out of a roll 60, the flexible films are superposed with each other in succession at the fed out position and the superposed parts are sealed circularly except a part thereof and the viscous liquid is injected to fill the inside from an opening 64 not sealed yet and then, the opening 64 is sealed to realize an enclosed state in which the viscous liquid is charged. These overlapping process, a circular seal process except a part, the injecting and filling process of the viscous fluid, and a seal process of the opening 64 are successively executed to continuously produce a film damper 16.