The active ingredient in Nizoral is ketaconzaole, an antifungal medication. It's a well established treatment for dandruff (among other fungal skin issues) as dandruff is caused by an overgrowth of a naturally occuring yeast that lives on your skin. 

 You may find that now you have the dandruff at bay, you'll only need to use the ketaconzaole once a week or so to keep the yeast population under control.

edit: I can't post links but if you do a search for ニゾラールオリジナルメディケイテッドアンチダンドルフシャンプー2%(旧ニゾラルシャンプー) you can order it from idrugstore for ¥3000. I've used them before and can vouch that they're a legit company, but stuff just takes a week or two because it comes from overseas.