An image forming device that forms an image by transferring a toner image formed by developing an electrostatic latent image formed on a photoreceptor from the photoreceptor to an intermediate transfer body by applying a transfer bias to a transfer member and putting the transfer member in contact with the intermediate transfer body. The device includes: a current supplier selectively supplying a first constant current and a second constant current to the transfer member; a first voltage acquirer acquiring a first voltage occurring between the transfer member and the intermediate transfer body while the transfer member is supplied with the first constant current; a second voltage acquirer acquiring a second voltage occurring between the transfer member and the photoreceptor while the transfer member is supplied with the second constant current; and a thickness acquirer acquiring a value indicating photoreceptor layer thickness of the photoreceptor by using the first and second voltages.