A protection apparatus adapted to receive electric power from a power supply and to supply electric power to a load via an electric wire connected to the protection apparatus, the protection apparatus comprising: a switch unit which is adapted to interrupt a current flow between the power supply and the load; a current detection unit which detects current flowing into the electric wire; a temperature calculation unit; an over-current determination unit which determines to be over-current; and an interruption control unit which interrupts the switch unit when the over-current determination unit determines to be over-current; characterized in that said temperature calculation unit is provided with a pseudo heat generation temperature calculation expression and a pseudo heat dissipation temperature calculation expression in which a pseudo conductor resistance r* is used in said calculation expressions for calculating heat generation amount and heat dissipation amount of the electric wire instead of an actual conductor resistance r of the electric wire set and in which a pseudo heat resistance R* is used in said calculation expressions instead of an actual heat resistance R of the electric wire, the temperature calculation unit calculating an estimation temperature T2 of the electric wire by using the pseudo heat generation temperature calculation expression, the pseudo heat dissipation temperature calculation expression and the current detected by the current detection unit, the pseudo conductor resistance r* has a value simulating current ● interruption time characteristics of a fuse capable of being used to protect the electric wire, the pseudo heat resistance R* has a value simulating the current ● interruption time characteristics of the fuse, wherein the pseudo conductor resistance r* is larger than the conductor resistance r of the electric wire and the pseudo heat resistance R* is smaller than the heat resistance R of the electric wire; and said over-current determination unit determines to be over-current when the estimation temperature calculated by the temperature calculation unit reaches an upper limit temperature. a switch unit which is adapted to interrupt a current flow between the power supply and the load; a current detection unit which detects current flowing into the electric wire; a temperature calculation unit; an over-current determination unit which determines to be over-current; and an interruption control unit which interrupts the switch unit when the over-current determination unit determines to be over-current; characterized in that said temperature calculation unit is provided with a pseudo heat generation temperature calculation expression and a pseudo heat dissipation temperature calculation expression in which a pseudo conductor resistance r* is used in said calculation expressions for calculating heat generation amount and heat dissipation amount of the electric wire instead of an actual conductor resistance r of the electric wire set and in which a pseudo heat resistance R* is used in said calculation expressions instead of an actual heat resistance R of the electric wire, the temperature calculation unit calculating an estimation temperature T2 of the electric wire by using the pseudo heat generation temperature calculation expression, the pseudo heat dissipation temperature calculation expression and the current detected by the current detection unit, the pseudo conductor resistance r* has a value simulating current ● interruption time characteristics of a fuse capable of being used to protect the electric wire, the pseudo heat resistance R* has a value simulating the current ● interruption time characteristics of the fuse, wherein the pseudo conductor resistance r* is larger than the conductor resistance r of the electric wire and the pseudo heat resistance R* is smaller than the heat resistance R of the electric wire; and said over-current determination unit determines to be over-current when the estimation temperature calculated by the temperature calculation unit reaches an upper limit temperature.