Masked Bannerfish (Heniochus monoceros) オニハタタテダイ
An adult Masked Bannerfish has the forward white bar through the body going to the spine on the dorsal fin. There are similar fish.
A younger Masked Bannerfish has a little different shape.
Head on view of the Masked Bannerfish so you can see the mask.
Four different butterflyfish here and a Morish Idol. The three that are similar are the Singular Bannerfish, Masked bannerfish and the Morish Idol (not even a Butterflyfish). See if you can tell the difference between them.