In a case of a normal operation, a battery controller (22) assumes a normal operation mode and permits charging and discharging of a battery module (21). Upon receiving a discharge command from a vehicle body controller (17) for completely discharging the battery module (21), the battery controller (22) assumes a discharge mode to discharge the battery module (21) to reach a safe voltage level and store discharge information. Upon reading out the discharge information, the battery controller (22) assumes a discharge prohibition mode to prohibit charging and discharging of the battery module (21).