A protection apparatus of a load circuit, comprising: a timer configured to count an elapsed time of a current flowing through a load via an electric wire; a current detection unit configured to detect the current; a switch portion configured to switch supply and interruption of the current; a temperature estimation unit configured to estimate a temperature of the electric wire based on a pseudo-temperature arithmetic expression; and a breaking control unit configured to break the switch portion when the temperature estimated by the temperature estimation unit has reached an allowed temperature of the electric wire, the protection apparatus being further characterized in that: the pseudo-temperature arithmetic expression is set in such a manner that, in a temperature arithmetic expression of the electric wire, the temperature arithmetic expression using the elapsed time counted by the timer, the current detected by the current detection unit, and a heat capacity and conductor resistance of the electric wire, a pseudo-heat capacity smaller than the heat capacity of the electric wire is assigned to the heat capacity, and a pseudo-conductor resistance larger than the conductor resistance of the electric wire is assigned to the conductor resistance, wherein the pseudo-conductor resistance and the pseudo-heat capacity are set so that current/breaking time characteristics which are based on the pseudo-temperature arithmetic expression is located between minimum current/breaking time characteristics of a fuse protecting the electric wire and maximum current/breaking time characteristics of the fuse. a timer configured to count an elapsed time of a current flowing through a load via an electric wire; a current detection unit configured to detect the current; a switch portion configured to switch supply and interruption of the current; a temperature estimation unit configured to estimate a temperature of the electric wire based on a pseudo-temperature arithmetic expression; and a breaking control unit configured to break the switch portion when the temperature estimated by the temperature estimation unit has reached an allowed temperature of the electric wire, the protection apparatus being further characterized in that: the pseudo-temperature arithmetic expression is set in such a manner that, in a temperature arithmetic expression of the electric wire, the temperature arithmetic expression using the elapsed time counted by the timer, the current detected by the current detection unit, and a heat capacity and conductor resistance of the electric wire, a pseudo-heat capacity smaller than the heat capacity of the electric wire is assigned to the heat capacity, and a pseudo-conductor resistance larger than the conductor resistance of the electric wire is assigned to the conductor resistance, wherein the pseudo-conductor resistance and the pseudo-heat capacity are set so that current/breaking time characteristics which are based on the pseudo-temperature arithmetic expression is located between minimum current/breaking time characteristics of a fuse protecting the electric wire and maximum current/breaking time characteristics of the fuse. the pseudo-temperature arithmetic expression is set in such a manner that, in a temperature arithmetic expression of the electric wire, the temperature arithmetic expression using the elapsed time counted by the timer, the current detected by the current detection unit, and a heat capacity and conductor resistance of the electric wire, a pseudo-heat capacity smaller than the heat capacity of the electric wire is assigned to the heat capacity, and a pseudo-conductor resistance larger than the conductor resistance of the electric wire is assigned to the conductor resistance, wherein the pseudo-conductor resistance and the pseudo-heat capacity are set so that current/breaking time characteristics which are based on the pseudo-temperature arithmetic expression is located between minimum current/breaking time characteristics of a fuse protecting the electric wire and maximum current/breaking time characteristics of the fuse.