This is a male. Formerly considered conspecific with the Common Scoter (M. nigra), this has an extensive North American distribution with widely separated breeding populations in Alaska and Northern Quebec. However another population breeds in the Russian Far East and these are presumably the source of the wintering birds along the coast of Japan. Canon PowerShot SX60.
References:
Bordage, D. and J. L. Savard (2011). Black Scoter (Melanitta americana), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.177
del Hoyo, J., Collar, N. & Kirwan, G.M. (2019). Black Scoter (Melanitta americana). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/467115 on 21 May 2019).