To provide a technology to accurately obtain a maximum temperature and a minimum temperature of a battery.

A battery temperature detector 1 is equipped with temperature measurement means 21, use state determination means 22, and maximum/minimum temperature calculation means 23. The temperature measurement means 21 measures a temperature of the battery. The use state determination means 22 determines whether the use state of the battery is (A) in the running/charging time or (B) in the time of using an equalization circuit or (C) in the time of cooling by a fan, and transmits the use state to the maximum/minimum temperature calculation means 23. The maximum/minimum temperature calculation means 23 includes a storage means 24 storing an offset value or a correction coefficient for correcting the offset value to calculate the maximum temperature and minimum temperature based on the measured temperature received from the temperature measurement means 21, a fan rotation speed, the number of the equalized batteries (the number of the batteries under equalization process), and the use state of the battery received from the use state determination means 22. The maximum/minimum temperature calculation means 23 outputs the calculated maximum and minimum temperatures.