A temperature detection portion 41 captures temperature detection values detected by a plurality of temperature sensors T disposed at prescribed positions of a battery portion 2 with a prescribed period, to detect the state of the battery portion 2. An abnormality judgment portion 42 compares the temperature detection values detected by the temperature detection portion 41 with a threshold value determined in advance, to judge whether the state of the battery portion 2 is abnormal. In a case where the abnormality judgment portion 42 judges that the battery portion 2 is abnormal, a charge/discharge control portion 43 causes a switching element 5 to operate in an on/off sequence set in advance, and then turns off the switching element 5.