An image forming apparatus comprising: a main body; a plurality of developing units (30C, 30M, 30Y, and 30K) separably installed at the main body, each of the plurality of developing units (30C, 30M, 30Y, and 30K) being configured to develop an electrostatic latent image into a visible image with developer; and a transfer unit arranged to transfer the developer of each developing unit (30C, 30M, 30Y, and 30K) developed into the visible image to a printing medium, wherein: each of the plurality of developing units (30C, 30M, 30Y, and 30K) comprises a photosensitive body on which the electrostatic latent image is formed, and a developing unit housing at which the photosensitive body is rotatably mounted; the transfer unit comprises an intermediate transfer belt to which developer on the photosensitive body is transferred, first transfer rollers, each of the first transfer rollers transferring the developer from the photosensitive body to the intermediate transfer belt, a transfer unit frame at which the first transfer rollers are movably mounted, and a second transfer roller to transfer developer from the intermediate transfer belt to the printing medium; and the developing unit housing comprises position regulating guides, each of the position regulating guides, when each developing unit (30C, 30M, 30Y, and 30K) is installed at the main body, supporting a shaft of each first transfer roller so that the intermediate transfer belt comes into contact with the photosensitive body at a uniform angle; wherein: the transfer unit comprises movable guides, each of the movable guides rotatably supporting the shaft of each first transfer roller while being movably mounted at the transfer unit frame; characterised in that each of the position regulating guides comprises a position regulating groove arranged to receive each movable guide; and in that each of the developing units (30C, 30M, 30Y, and 30K) comprises rotational hooks, each of the rotational hooks being rotatably installed at each position regulating guide to be latched at each movable guide during rotation of the rotational hook, thereby allowing the movable guide to be maintained in a state received within the position regulating groove. a main body; a plurality of developing units (30C, 30M, 30Y, and 30K) separably installed at the main body, each of the plurality of developing units (30C, 30M, 30Y, and 30K) being configured to develop an electrostatic latent image into a visible image with developer; and a transfer unit arranged to transfer the developer of each developing unit (30C, 30M, 30Y, and 30K) developed into the visible image to a printing medium, wherein: each of the plurality of developing units (30C, 30M, 30Y, and 30K) comprises a photosensitive body on which the electrostatic latent image is formed, and a developing unit housing at which the photosensitive body is rotatably mounted; the transfer unit comprises an intermediate transfer belt to which developer on the photosensitive body is transferred, first transfer rollers, each of the first transfer rollers transferring the developer from the photosensitive body to the intermediate transfer belt, a transfer unit frame at which the first transfer rollers are movably mounted, and a second transfer roller to transfer developer from the intermediate transfer belt to the printing medium; and the developing unit housing comprises position regulating guides, each of the position regulating guides, when each developing unit (30C, 30M, 30Y, and 30K) is installed at the main body, supporting a shaft of each first transfer roller so that the intermediate transfer belt comes into contact with the photosensitive body at a uniform angle; wherein: each of the plurality of developing units (30C, 30M, 30Y, and 30K) comprises a photosensitive body on which the electrostatic latent image is formed, and a developing unit housing at which the photosensitive body is rotatably mounted; the transfer unit comprises an intermediate transfer belt to which developer on the photosensitive body is transferred, first transfer rollers, each of the first transfer rollers transferring the developer from the photosensitive body to the intermediate transfer belt, a transfer unit frame at which the first transfer rollers are movably mounted, and a second transfer roller to transfer developer from the intermediate transfer belt to the printing medium; and the developing unit housing comprises position regulating guides, each of the position regulating guides, when each developing unit (30C, 30M, 30Y, and 30K) is installed at the main body, supporting a shaft of each first transfer roller so that the intermediate transfer belt comes into contact with the photosensitive body at a uniform angle; wherein: the transfer unit comprises movable guides, each of the movable guides rotatably supporting the shaft of each first transfer roller while being movably mounted at the transfer unit frame; characterised in that each of the position regulating guides comprises a position regulating groove arranged to receive each movable guide; and in that each of the developing units (30C, 30M, 30Y, and 30K) comprises rotational hooks, each of the rotational hooks being rotatably installed at each position regulating guide to be latched at each movable guide during rotation of the rotational hook, thereby allowing the movable guide to be maintained in a state received within the position regulating groove.