About 40 other people aboard the bus were injured when the vehicle struck a sound barrier along the Kanetsu Expressway in Fujioka, Kyodo News reported.
Police and the bus company told the news service the bus had left Kanazawa at 10 p.m. Saturday, the start of Japan's so-called Golden Week spring holiday season.
The Daily Yomiuri reported 13 passengers were severely injured, and another 25 received minor injuries. The newspaper said Jiji Press reported six of those killed were believed to be women in their 20s and the seventh a middle-age man.
The newspaper said Gunma prefectural police were investigating whether the 43-year-old bus driver was negligent.