@ Bonnybrook, davidships & lappino: yes, that seems to be indeed Kasusho Maru, IMO 92075455. Thanks for the ID.

This is the only tug that worked around Nagasaki in 2014 with this specific bridge layout, so I second David's opinion that your call is pretty certain, Bonnybrook.

I've uploaded the side view of "Kakusho Maru" from Nagasaki now (just like Gena did on several occasions before).

Cheers

Vlad

Earlier cached version of that web-page from 2/2016 has the same tugs only.
Equasis seems more up-to-date:
MANUZURU MARU (IMO 8408583)(wrongly labelled on website) now listed as sold "unknown" 2/2017;
KOHKAKU MARU (IMO 9800582) newbuilding purchased 2017.

I think yours is a pretty certain call, Bonnybrook.
KAKUSHO MARU - IMO 9207455

This is a tug of the company, Kowa Kogyo. The web page https://advectionfog.net/tug/kowa_01.html introduces
their tugs and the 鶴翔丸 (Kakusho Maru) is the only one shown that matches the one in Dirk's picture. As I don't know if the web page shows the whole fleet, I won't declare a sure ID but there's a strong chance the tug here is the Kakusho Maru.