An invented image forming apparatus has a fixing device for stably fixing developer on a medium. The fixing device includes a heating member, an endless belt member subject to heating provided from the heating member, a tension member tensioning the belt member, a pressure member forming a pressurized contact nipping portion via the belt member, a supporting member supporting the pressure member, a pushing member pushing the pressure member and the belt member, a fixing member pushing the pressure member and the belt member, a medium guide guiding move of the medium in a direction toward the pressurized contact nipping portion, and a pressure member moving mechanism for moving a position of the pressure member in the supporting member. The image forming apparatus further includes a controller controlling the position of the pressure member via the pressure member moving mechanism.