Apparently they don't use BMI but waistline measurements instead. Regardless of the method, the Japanese government is applying fines for being overweight.
http://www2.tbo.com/content/2008/jun/13/na-waist-not-japan-measures-up-in-fight-against-ob/
http://www2.tbo.com/content/2008/jun/13/na-waist-not-japan-measures-up-in-fight-against-ob/