There have always been all-female bands with moderate popularity in the indies scene in Japan. It's very rare that they stick together and break through though.
That said, this group looks significantly more "idol" than most of the other all-girl bands I've seen...Was really surprised to learn that they're not an Avex band. However, their label, Spinning, is primarily an independent label for foreign artists and they specialize in female vocalists. They've started a new label specifically for all-girl Japanese metal bands called Bright Star Records but I think this band is their only artist. The Japanese music industry is very weird and sketchy with money. I'm really interested to know who is controlling the money here (like Dynamite Tommy pretty much does with VK...).
Check out the early years of HIGH and MIGHTY COLOR. 2004-2008 when they had Mākii. She had a voice I really liked and the band had really catchy songs.
They might be a bit too Pop for a lot of people here though.
Their cover of Luna Sea's "Rosier" is immense.