In a press release on Thursday, Nokia explains his departure is due to inability to do the job full-time.
Replacing Kallasvuo, Jesper Ovesen takes the position of Executive Chairman of the NSN board of directors immediately.
The 52-year-old Dane has earlier served as the Chief Financial Officer at the Danish telecommunications group TDC and at toymaker Lego.
Now 58, Kallasvuo has worked at Nokia since 1980. He served as its CEO from 2006 until Stephen Elop took the post a year ago.
