Filling of a powder particle material is executed for a flexible container bag (hereinafter, referred to as “flex container bag”) by executing a powder particle material filling step of filling the flex container bag with the powder particle material by supplying the powder particle material to the flex container bag, and a tapping step of executing a lifting operation of lifting up the flex container bag filled with the powder particle material to cause the flex container bag to be distant from a floor surface or a base bed thereabove and a dropping operation of dropping the flex container bag by releasing the lifting by the lifting operation to cause the flex container bag to collide with the floor surface or the base bed. The handling property of the flex container bag filled with the powder particle material can further be improved.