A method of developing electrostatic latent images in a recording apparatus by forming a layer of developer containing toner particles on a conveying means, the thickness of said layer being smaller than the space existing between the image forming surface and the conveying means. The developer is transferred to the image forming surface by an alternating bias voltage. A detecting means determines the toner concentration in the developer; and the developer conditions are controlled in response to the detection of the toner concentration so that image density remains substantially constant despite change in toner concentration.