As an effective method to protect against damage from a terrorist attack or accidental explosion, important facilities can be constructed below ground. To improve the blast protection performance, a mitigation system consisting mainly of expanded polystyrene (EPS) and sand can be laid above an underground structure. In this study, the effects of a metal sheet which protects an EPS plate from explosion on blast mitigation were investigated experimentally. From the test results, in the bending stiffness per width of a metal sheet approximately 30kNmm or less, the blast pressure was decreased and dispersed with the increase of the bending stiffness per width of the metal sheet.