A method of determining a pseudo heat capacity Cth* used in a protection apparatus for a load circuit, which protects the load circuit when a temperature of an electric wire rises in the load circuit which supplies a power output from a power supply to a load to drive the load, the protection apparatus comprising: a count unit which is adapted to count an elapsed time; a current detection unit which is adapted to detect a current flowing through the electric wire of the load circuit; a switch unit which is adapted to switch between connection and interruption of the load circuit; a temperature calculation unit which is adapted to set a pseudo heat capacity smaller than a heat capacity of the electric wire used in the load circuit so as to simulate the fusing characteristics of a fuse, and to calculate the temperature of the electric wire with reference to an arithmetic expression of heat generation amount of the electric wire, an arithmetic expression of heat radiation amount of the electric wire, the time counted by the count unit, and the pseudo heat capacity; and an interruption control unit which is adapted to interrupt the switch unit when the temperature of the electric wire calculated by the temperature calculation unit exceeds a preset threshold temperature, the method including the following steps: (i) carrying a current through a fuse having desired fusing characteristics to acquire current/time characteristics data indicating a relationship between a conducting current and a fusing time; (ii) calculating a maximum current Imax which can be continuously conducted through the fuse from the current/time characteristics data; (iii) calculating a temperature threshold ΔTmax at the time of continuously conducting the maximum current Imax through a downstream electric wire of a protection circuit by the following expression: ΔTmax = Rth × Ron × Imax 2; where Rth indicates a heat resistance per unit length of the electric wire, and Ron indicates an electric resistance per unit length of the electric wire; (iv) calculating the pseudo heat capacity Cth* by the following expression as a function of the temperature threshold ΔTmax: C th * = − t R th ln 1 − Δ T max R th R on I 2; and (v) setting the calculated pseudo heat capacity Cth* as said pseudo heat capacity by the temperature calculation unit in said protection apparatus. a count unit which is adapted to count an elapsed time; a current detection unit which is adapted to detect a current flowing through the electric wire of the load circuit; a switch unit which is adapted to switch between connection and interruption of the load circuit; a temperature calculation unit which is adapted to set a pseudo heat capacity smaller than a heat capacity of the electric wire used in the load circuit so as to simulate the fusing characteristics of a fuse, and to calculate the temperature of the electric wire with reference to an arithmetic expression of heat generation amount of the electric wire, an arithmetic expression of heat radiation amount of the electric wire, the time counted by the count unit, and the pseudo heat capacity; and an interruption control unit which is adapted to interrupt the switch unit when the temperature of the electric wire calculated by the temperature calculation unit exceeds a preset threshold temperature, the method including the following steps: (i) carrying a current through a fuse having desired fusing characteristics to acquire current/time characteristics data indicating a relationship between a conducting current and a fusing time; (ii) calculating a maximum current Imax which can be continuously conducted through the fuse from the current/time characteristics data; (iii) calculating a temperature threshold ΔTmax at the time of continuously conducting the maximum current Imax through a downstream electric wire of a protection circuit by the following expression: ΔTmax = Rth × Ron × Imax 2; where Rth indicates a heat resistance per unit length of the electric wire, and Ron indicates an electric resistance per unit length of the electric wire; (iv) calculating the pseudo heat capacity Cth* by the following expression as a function of the temperature threshold ΔTmax: C th * = − t R th ln 1 − Δ T max R th R on I 2; and (v) setting the calculated pseudo heat capacity Cth* as said pseudo heat capacity by the temperature calculation unit in said protection apparatus. (i) carrying a current through a fuse having desired fusing characteristics to acquire current/time characteristics data indicating a relationship between a conducting current and a fusing time; (ii) calculating a maximum current Imax which can be continuously conducted through the fuse from the current/time characteristics data; (iii) calculating a temperature threshold ΔTmax at the time of continuously conducting the maximum current Imax through a downstream electric wire of a protection circuit by the following expression: ΔTmax = Rth × Ron × Imax 2; where Rth indicates a heat resistance per unit length of the electric wire, and Ron indicates an electric resistance per unit length of the electric wire; (iv) calculating the pseudo heat capacity Cth* by the following expression as a function of the temperature threshold ΔTmax: C th * = − t R th ln 1 − Δ T max R th R on I 2; and (v) setting the calculated pseudo heat capacity Cth* as said pseudo heat capacity by the temperature calculation unit in said protection apparatus.