This is a male with a broad black ventral stripe extending to its undertail coverts. This species lacks the yellow underparts of (Northern) Great Tit but has a green back as seen here rather than a gray back as in Cinereous (Southern Great) Tit. Taxonomy is complex and unsettled. Traditionally included as a race of the Great Tit (Parus major) but split into a separate species called Japanese Tit (aka Eastern Great Tit) by Clement/eBird and IOC. However H&M4 treat this bird as a race of Cinereous Tit (Parus cinereus), aka Southern Great Tit now also split from Great Tit. HBW lumps them all as races of Great Tit "perpetuating an old and increasingly obsolete taxonomy" (Päckert & Martens 2008). Within the Japanese Tit, four subspecies occur in Japan. This is the widespread nominate race. Three others are endemic to the Ryukyu Islands. They are darker and have less green on their back. Canon PowerShot SX50 HS.
References:
Brazil, M. (2009). Birds of East Asia. Princeton Univ. Press.
Collar, N.J. & Pilgrim, J.D. (2007). Species-level changes proposed for Asian birds, 2005-2006. Birding Asia, 8:14-30.
Gosler, A., Clement, P. & Christie, D.A. (2017). Great Tit (Parus major). 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 http://www.hbw.com/node/59900 on 22 June 2017).
Harrap, S. & Quinn. D. (1995) Chickadees, Tits, Nuthatches & Treecreepers. Princeton Univ. Press.
Päckert, M., Martens, J., Eck, S., Nazarenko, A.A., Valchuk, O.P., Petri, B. & Veith, M. (2005) The Great Tit (Parus major) - a misclassified ring species. Biol. J. Linn. Soc. 86(2): 153174.
Päckert, M., & Martens, J. (2008). Taxonomic pitfalls in titscomments on the Paridae chapter of the Handbook of the Birds of the World. Ibis, 150(4), 829-831.