A novel stabilizing liquid using air foam rather than bentonite clay slurry, i.e. air foam suspension, is employed in order to stabilize the trench wall surface during a diaphragm wall excavation. Air foam suspension is used to reduce the construction costs of working on an underground diaphragm wall. Air foam suspension is created by mixing the excavated soil with air foam made from a surface-active chemical agent. The performance of air foam suspension depends on its density and consistency, that is, its table flow value (TF). By comparing the trench stabilization capacity of air foam suspension with that of bentonite clay slurry in model tests, the appropriate performance of air foam suspension was confirmed. The cost evaluation of using an air foam suspension for a diaphragm wall excavation is presented.