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 first temperature arithmetic operation unit configured to estimate a temperature of the electric wire based on a first pseudo-heat generation temperature arithmetic expression and a first pseudo-heat radiation temperature arithmetic expression; a second temperature arithmetic operation unit configured to estimate the temperature of the electric wire based on a second pseudo-heat generation temperature arithmetic expression and a second pseudo-heat radiation temperature arithmetic expression; an overcurrent determination unit configured to determine that, when the current detected by the current detection unit is smaller than a preset predetermined threshold current, and an estimated temperature calculated by the first temperature arithmetic operation unit has reached a first allowed temperature, the current thus detected is an overcurrent, and when the current detected by the current detection unit is equal to or larger than the threshold current, and an estimated temperature calculated by the second temperature arithmetic operation unit has reached a second allowed temperature, the current thus detected is an over current; and a breaking control unit configured to break the switch portion when the overcurrent determination unit has determined that the detected current is the overcurrent, wherein the first pseudo-heat generation temperature arithmetic expression and the first pseudo-heat radiation temperature arithmetic expression are 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 first pseudo-heat capacity is assigned to the heat capacity, the second pseudo-heat generation temperature arithmetic expression and the second pseudo-heat radiation temperature arithmetic expression are set in such a manner that a second pseudo-heat capacity is assigned to the heat capacity in the temperature arithmetic expression of the electric wire, and the first and second pseudo-heat capacities are different from each other. 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 first temperature arithmetic operation unit configured to estimate a temperature of the electric wire based on a first pseudo-heat generation temperature arithmetic expression and a first pseudo-heat radiation temperature arithmetic expression; a second temperature arithmetic operation unit configured to estimate the temperature of the electric wire based on a second pseudo-heat generation temperature arithmetic expression and a second pseudo-heat radiation temperature arithmetic expression; an overcurrent determination unit configured to determine that, when the current detected by the current detection unit is smaller than a preset predetermined threshold current, and an estimated temperature calculated by the first temperature arithmetic operation unit has reached a first allowed temperature, the current thus detected is an overcurrent, and when the current detected by the current detection unit is equal to or larger than the threshold current, and an estimated temperature calculated by the second temperature arithmetic operation unit has reached a second allowed temperature, the current thus detected is an over current; and a breaking control unit configured to break the switch portion when the overcurrent determination unit has determined that the detected current is the overcurrent, wherein the first pseudo-heat generation temperature arithmetic expression and the first pseudo-heat radiation temperature arithmetic expression are 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 first pseudo-heat capacity is assigned to the heat capacity, the second pseudo-heat generation temperature arithmetic expression and the second pseudo-heat radiation temperature arithmetic expression are set in such a manner that a second pseudo-heat capacity is assigned to the heat capacity in the temperature arithmetic expression of the electric wire, and the first and second pseudo-heat capacities are different from each other.