This may just be normal for your car Steve. Some vehicles are equipped with an ammeter that indicates charge or discharge, current passing in or out of the battery. With all of the electrical equipment turned ON, and the engine idling, the guage needle may show a discharge condition. At fast idle or normal driving speeds the needle should stay on the charge side of the guage with the charge state of the battery determining how far over the needle will swing.
Some cars are equipped with a voltmeter that indicates battery voltage with the key on and the engine off, and alternator output when the engine is running.
If you have Under Drive Pulleys, it will cause the alternator to have a lower than normal output at idle.
Rollo's and Grumpy's advice is also correct... You have to give the wiring, cables, and connections a good looking over on cars that are this age. Particularly the ground connections.
Id tell you to go to AutoZone and have it all tested but since you're in Japan... There must be a place on base to have it checked out or somewhere.
You'll figure that part out.