A fixing device includes: a belt unit that includes a first roller member, a second roller member provided above the first roller member, a belt member rotating while laid around these roller members, and a positioning member rotatably holding the first roller member and used for positioning the first roller member; a pressure member provided below the first roller member and coming into pressure-contact with an outer peripheral surface of the belt member at a position where the pressure member faces the first roller member, thereby to form a fixing pressure portion; a first housing that supports the positioning member and includes a concave portion opening toward a lateral direction and a sloping portion sloping downward from a lower edge portion of the concave portion; and a second housing pressing the positioning member to fit into the concave portion, along with a closing operation with respect to the first housing.