A fibrous-mass product is provided which has both water retentivity and strength, and in addition, good air permeability, and can be used as an artificial soil particle. A fibrous-mass product 1 includes a core portion 3 which is an aggregate of fibers 2, and a water permeable membrane 4 which covers the core portion 3. The water permeable membrane 4 has porous structure, and covers at least 40% of an outer surface portion of the core portion 3. The core portion 3 has a water retention material. Ion exchange capability is imparted to at least one of the core portion 3 and the water permeable membrane 4. The fibrous-mass product 1 has a grain size of 1-10 mm. The amount of moisture retained in the fibrous-mass product 1 is adjustable between a wet state in which the core portion 3 is saturated with moisture and an air permeable state in which the core portion 3 is not saturated with moisture.