To make it possible to set the range of the charging rate of batteries and the start/finish of generator drive according to the temperature change of the batteries, by controlling the generation of the generator according to the output of a temperature detecting means for detecting the temperatures of the batteries.

When a battery monitor 23 judges that charging of batteries 10 is necessary, it detects the temperatures of the batteries 10 by a temperature sensor 11, sets a charging rate and times for generation start/finish of a generator 15 in accordance with the temperatures of the batteries 10, and outputs signals to a generation controller 20 so as to charge the batteries 10 on these conditions. At this time, the battery monitor 23 sets the charging rate of the batteries and times for generation start/finish on the basis of the temperature of a battery having the lowest temperature in the batteries 10. A generation controller 20 outputs signals to an ECU 21 and drives an engine 16, and generates power by driving the generator 15. Next the generation controller 20 charges the batteries 10 on conditions set by the battery monitor 23.