To provide a filling device for a flexible container bag in which the container box can be easily reversed, the stored content can be taken out and filled in the flexible container bag.

This filling device is constructed such that the lower end of a container box 11 sliding on a slant surface is engaged on the slant surface of a slant part in a reverse trapezoidal shaped hopper having a slant part 2 at one side, there is provided an engagement member 5 for reversing the container box 11 to take out stored content 12 stored in the container box and further the bottom side of the hopper is provided with an opening part for use in discharging the taken-out stored content.