The authors examined the behaviors of fumigant gases with Siporex, an autoclaved lightweight concrete; Siporex has been recently used as a structural member for repositories. Results: (1) The permeability of the fumigant gases, methyl bromide (MB) and ethylene oxide (EO), through a 100mm thick Siporex piece is very rapid. (2) Adsorption of fumigant gases in Siporex is very little. One day after the fumigation no MB and 4.3ppm of EO remained. (3) Siporex pieces which volatilized 80-85ppm of MB and 2, 000ppm of EO just after fumigation into a 5-liter closed space volatilized only 2ppm of MB and 100ppm of EO into the same space after left in air for one day. (4) As Siporex adsorbs only a small amount of fumigant gases and desorbs them in one or two days, the fumigation for storage buildings must be conducted by the covered fumigation method. See also AATA 18-891 for abstract of Part 1. -- AATA