A protection device for a load circuit including a power supply and a load, the protection device comprising: a switch; a conductor connectable between the power supply and the load; a current detection device arranged to detect and measure a current in the conductor; a first temperature estimation device arranged to estimate a rising temperature of the conductor, when the current is detected by the current detection device; a second temperature estimation device arranged to estimate a falling temperature of the conductor when the current detection device fails to detect the current or detects that the current is being decreased; a third temperature estimation device arranged to estimate an arc-induced rising temperature of the conductor that is induced by arcing that occurs just before the current returns to a normal current; a fourth temperature estimation device arranged to estimate a present temperature of the conductor based on the estimated rising, falling, and arc-induced rising temperatures; a temperature determination device arranged to determine whether the present temperature of the conductor estimated by the fourth temperature estimation device exceeds a predetermined threshold temperature; a shutoff control device arranged to shut off power supplied to the load circuit during operation of the protection device by opening the switch when the temperature determination device determines that the present temperature of the conductor exceeds the predetermined threshold temperature; and a memory arranged to store each of a plurality of values of a temporary current that flows through the conductor just before the current returns to the normal current in association with a respective temperature increment that is caused by the temporary current, wherein the first temperature estimation device is arranged to estimate the rising temperature based on both the current measured by the current detection device and thermal properties of the conductor, the second temperature estimation device is arranged to estimate the falling temperature of the conductor based on the thermal properties of the conductor; and the third temperature estimation device is arranged to estimate the arc-induced rising temperature by finding a stored temperature increment that corresponds to the measured current just before the current returns to the normal current. a switch; a conductor connectable between the power supply and the load; a current detection device arranged to detect and measure a current in the conductor; a first temperature estimation device arranged to estimate a rising temperature of the conductor, when the current is detected by the current detection device; a second temperature estimation device arranged to estimate a falling temperature of the conductor when the current detection device fails to detect the current or detects that the current is being decreased; a third temperature estimation device arranged to estimate an arc-induced rising temperature of the conductor that is induced by arcing that occurs just before the current returns to a normal current; a fourth temperature estimation device arranged to estimate a present temperature of the conductor based on the estimated rising, falling, and arc-induced rising temperatures; a temperature determination device arranged to determine whether the present temperature of the conductor estimated by the fourth temperature estimation device exceeds a predetermined threshold temperature; a shutoff control device arranged to shut off power supplied to the load circuit during operation of the protection device by opening the switch when the temperature determination device determines that the present temperature of the conductor exceeds the predetermined threshold temperature; and a memory arranged to store each of a plurality of values of a temporary current that flows through the conductor just before the current returns to the normal current in association with a respective temperature increment that is caused by the temporary current, wherein the first temperature estimation device is arranged to estimate the rising temperature based on both the current measured by the current detection device and thermal properties of the conductor, the second temperature estimation device is arranged to estimate the falling temperature of the conductor based on the thermal properties of the conductor; and the third temperature estimation device is arranged to estimate the arc-induced rising temperature by finding a stored temperature increment that corresponds to the measured current just before the current returns to the normal current.