I was not able to find a bus that goes from Shibukawa Station to Lake Haruna. The buses only as far as Ikaho Onsen.But there is a bus from Takasaki Station to Lake Haruna, operated by Gunma Bus. The one way trip takes 90 minutes and costs 1270 Yen.
The timetable is here (first line is Takasaki, last line is Lake Haruna):
http://sys.busnet-gunma.jp/cgi-bin/search/timetable.cgi?itm_num=189&area=3&in=&out=