An image forming device includes an electrostatic latent image carrier on which an electrostatic latent image is formed, a main frame which supports the electrostatic latent image carrier and a developing unit. The developing unit includes a developer carrier for supplying a developer to the electrostatic latent image carrier and a replenish chamber for replenishing the developer in the developer carrier. The developing unit is inserted removably in the main frame. A pressing mechanism is provided on the main frame and presses a bottom surface of the developing unit in a pressing direction along a straight line orthogonal to a rotational center shaft of the electrostatic latent image carrier and a rotational center shaft of the developer carrier. The developing unit includes a guide member which is fit into one end part of the rotational shaft of the electrostatic latent image carrier and guides the developing unit to move along the straight line. The main frame includes a positioning long hole which is fit into another end part of the rotational shaft of the developer carrier and guides the developing unit to move along the straight line.