A plus-side main connection line (17) connects electrical equipment (11), (12) to a plus terminal (16A) of a battery (16). A plus-side isolating switch (18) is provided in the plus-side main connection line (17) to connect or disconnect the electrical equipment (11), (12) and or from the plus terminal (16A) of the battery (16). A plus-side auxiliary connection line (20) connects a communication terminal (15) to the plus terminal (16A) of the battery (16) in a position upstream of the plus-side isolating switch (18). A minus-side main connection line (22) connects a minus terminal (16B) of the battery (16) to ground. A minus-side isolating switch (23) is provided in the minus-side main connection line (22) to connect or disconnect the minus terminal (16B) of the battery (16) and or from the ground.