Immunization of human antibody-producing transgenic mice, which have been created using genetic engineering techniques, with AILIM molecule as an antigen resulted in various human monoclonal antibodies capable of binding to AILIM and capable of controlling a variety of biological reactions (for example, cell proliferation, cytokine production, immune cytolysis, cell death, induction of ADCC, etc.) associated with AILIM-mediated costimulatory signal (secondary signal) transduction. Furthermore, it has been revealed that the human monoclonal antibody is effective to treat and prevent various diseases associated with AILIM-mediated costimulatory signal transduction, being capable of inhibiting the onset and/or advancement of the diseases.