1. An image forming apparatus, comprising: an image retainer; a charger for electrically charging the image retainer; a latent image former for forming a latent image of a recording image corresponding to image information on the charged image retainer; a developer for developing the latent image of the recording image so as to form a toner image of the recording image, the developer including a developing sleeve by which developer is conveyed onto the image retainer; an electric bias for applying a developing bias voltage to the developer; a control for controlling the charger, the latent image former, the developer and the electric bias so as to form a patch image used as a test image on the image retainer under a patch image forming condition which is different from a recording image forming condition under which the toner image of the recording image is formed; an optical detector for detecting a density of the patch image and generating a detection output; and the control controlling at least one of the charger; the developer and the electric bias in accordance with the detection output of the optical detector so that a density of the toner image of the recording image formed on the image retainer satisfies a predetermined condition, the control adjusting a peripheral speed of the developing sleeve of the developer during formation of the patch image to be different from that during formation of the recording image.