For quantitative estimation of the intra-layer porous structure in the initial stage of landfill formation with municipal solid waste incineration (MSWI) bottom ash, the water absorption of individual MSWI bottom ash particles was measured under still-water, degassed, and agitated conditions. The ratio of the water absorption rate found for the still-water procedure to the effective absorption capacity which was the one under degassing was 35.2%. In the water flow experiment of a column filled with MSWI bottom ash, the true density of the bottom ash was higher after water flow than before, which indicated that dissolution of the soluble components of the bottom ash particle surfaces resulted in a loss of apparent particle volume that more than offset the accompanying weight loss. The volume-based water absorption rate found for the bottom ash particles following 50 mL/h water flow through the column, as a ratio to the effective absorption capacity was about 51.8% of the effective absorption capacity. In a landfill layer comprised of MSWI bottom ash, it was suggested that some regions of the ash particle interiors underwent almost no contact with water.