A predetermined amount of an ink absorber, which is a fiber assembly in which intersections between fibers are not fused together, is charged into a compression and insertion apparatus. A rectangular parallelepiped-shaped insertion block is moved to an upper surface portion of the absorber, and one face of the ink absorber is pushed by a side plate. As such, a part excluding the part of the ink absorber which faces the side plate is compressed and surrounded. In this state, the ink absorber is compressed by the side plate. The ink absorber is inserted into a tank case by making a bottom plate slide, thereby opening the bottom face of the ink absorber and by moving down the insertion block.