An apparatus for an electric power tool comprising: an internal temperature estimation unit comprising: a temperature detection device that is adapted to detect a temperature of a cell of a battery that is a power source of an electric power tool; an initial-value setting device which is adapted to read, when a discharge from the battery is started or when a charge to the battery is started, the detected temperature from the temperature detection device and is adapted to set the detected temperature as an initial value; and a temperature increase amount calculation device which is adapted to read, during the discharge from the battery or during the charge to the battery, the detected temperature from the temperature detection device and is adapted to calculate a temperature increase amount of the cell of the battery based on a latest value of the read detected temperature and the initial value set by the initial-value setting device, and wherein the internal temperature estimation unit is adapted to output the temperature increase amount calculated by the temperature increase amount calculation device, as an estimated value representing the internal temperature of the battery; a determination device that is adapted to determine whether or not the estimated value obtained by the internal temperature estimation unit and representing the internal temperature of the battery exceeds a predetermined set temperature for overheat determination; and a protection device that is adapted to protect the battery by suspending or limiting the discharge from the battery or the charge to the battery when the determination device determines that the estimated value of the internal temperature exceeds the set temperature; the apparatus for electric power tool being characterized in that: the apparatus for electric power tool further comprises a first set-temperature correction device that is adapted to correct the set temperature for use in the overheat determination of the battery by the determination device, based on at least one of charge and discharge records, an open voltage, and a remaining capacity of the battery, such that the set temperature becomes lower as the number of records of each of overdischarge and overcharge is larger, or as the open voltage or the remaining capacity is lower. an internal temperature estimation unit comprising: a temperature detection device that is adapted to detect a temperature of a cell of a battery that is a power source of an electric power tool; an initial-value setting device which is adapted to read, when a discharge from the battery is started or when a charge to the battery is started, the detected temperature from the temperature detection device and is adapted to set the detected temperature as an initial value; and a temperature increase amount calculation device which is adapted to read, during the discharge from the battery or during the charge to the battery, the detected temperature from the temperature detection device and is adapted to calculate a temperature increase amount of the cell of the battery based on a latest value of the read detected temperature and the initial value set by the initial-value setting device, and a temperature detection device that is adapted to detect a temperature of a cell of a battery that is a power source of an electric power tool; an initial-value setting device which is adapted to read, when a discharge from the battery is started or when a charge to the battery is started, the detected temperature from the temperature detection device and is adapted to set the detected temperature as an initial value; and a temperature increase amount calculation device which is adapted to read, during the discharge from the battery or during the charge to the battery, the detected temperature from the temperature detection device and is adapted to calculate a temperature increase amount of the cell of the battery based on a latest value of the read detected temperature and the initial value set by the initial-value setting device, and a determination device that is adapted to determine whether or not the estimated value obtained by the internal temperature estimation unit and representing the internal temperature of the battery exceeds a predetermined set temperature for overheat determination; and a protection device that is adapted to protect the battery by suspending or limiting the discharge from the battery or the charge to the battery when the determination device determines that the estimated value of the internal temperature exceeds the set temperature; the apparatus for electric power tool being characterized in that: the apparatus for electric power tool further comprises a first set-temperature correction device that is adapted to correct the set temperature for use in the overheat determination of the battery by the determination device, based on at least one of charge and discharge records, an open voltage, and a remaining capacity of the battery, such that the set temperature becomes lower as the number of records of each of overdischarge and overcharge is larger, or as the open voltage or the remaining capacity is lower. the apparatus for electric power tool further comprises a first set-temperature correction device that is adapted to correct the set temperature for use in the overheat determination of the battery by the determination device, based on at least one of charge and discharge records, an open voltage, and a remaining capacity of the battery, such that the set temperature becomes lower as the number of records of each of overdischarge and overcharge is larger, or as the open voltage or the remaining capacity is lower.