Packaging container-filler member, packaging container, contents-filled packaging container, and method of manufacturing contents-filled packaging container

A packaging container-filler member, comprising:

• a front-side sheet and a back-side sheet disposed to oppose each other;
• a first connecting sheet connecting outer edges of the front-side sheet and the back-side sheet on a first side in a first direction along the front-side sheet;
• a second connecting sheet connecting outer edges of the front-side sheet and the back-side sheet on a second side in the first direction;
• and
• a third connecting sheet connecting outer edges of the front-side sheet and the back-side sheet on a first side in a second direction along the front-side sheet, the second direction crossing the first direction, wherein
• the front-side sheet, the back-side sheet, the first connecting sheet, and the second connecting sheet constitute a cylindrical body whose cross section is rectangular when the cylindrical body has a three-dimensional shape, and
• the first connecting sheet, the second connecting sheet, and the third connecting sheet are each folded in an opposing direction in which the front-side sheet and the back-side sheet oppose each other.