The New Zealand Ministry of Foreign Affairs and Trade says no Kiwis were involved.
Chief cabinet secretary Yoshihide Suga told a news conference in Tokyo that 14 people were killed and 27 others injured in the single vehicle crash, The Guardian reports.
The crash happened about 2am (local time) north of Tokyo.
NHK World says 27 people are being treated in hospital for injuries.
The bus was carrying 40 skiers as it was heading to ski resorts in the Nagano prefecture, 2.5 kilometres south of Karuizawa station.
It is reported to have veered off the road before going through a guard rail and rolling down the hill.
There was no snow or ice on the road surface.
The transport ministry has launched an investigation into the accident, with inspectors being sent to the site.