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As far as reading material goes, a good starting point for the Air Law examination is the English language
Aeronautical Information Manual - Japan ( AIM - J)
Concentrate on section Ten.
It comes out twice each year and the next version is due out about now.
Cost is Y 3500.
Contact Point:
Japan Aircraft Pilot Association
1 - 18 - 14 Nishishimbashi
Minato-Ku
Tokyo
JAPAN
Tel: 03 3501 0433
The reality, however, of getting to the point of doing the actual exam is well covered by the previous posts. Sponsorship is essential.
Good Luck