A fixing device includes a heat assembly including an endless belt, a heat source, an attachment member having an attachment surface and first and second side surfaces, and a slide sheet having first and second end portions that are respectively fixed to a first fixing portion at the first side surface and a second fixing portion at the second side surface, the slide sheet having a larger length from the first end portion to the second end portion than a length of a path of the attachment member extending from the first fixing portion to the second fixing portion through the first side surface, the attachment surface, and the second side surface; and a pressure roller that presses the outer surface of the endless belt to the attachment surface.