The present invention provides an electret having high voltage characteristics. This electret sheet contains two synthetic resins that are immiscible in each other, and these synthetic resins form a phase separated structure. This electret sheet is characterized by having a synthetic resin sheet, in which the synthetic resins are formed from cross-linking by multifunctional monomers, electrostatically charged by infusion of an electric charge; therefore, at the interface parts for the two synthetic resins that are immiscible in each other, positive charges and negative charges are present in an apparent polarized state. If an outside force is applied to the electret sheet and the electret sheet is deformed, the relative positions of these positive charges and negative charges change, and an excellent electrical response arises with this change. Therefore, the electret sheet has superior piezoelectric properties.