An image-forming device forms an image having improving image quality by conveying a sheet in a nonuniform conveyance, and reduces a processing load required for calibrating a conveying length for the recording medium. In an image-forming device, a process for acquiring a theoretical conveying length for conveying a recording medium (S404) and for calibrating the theoretical conveying length (S409) is only executed for a large feed used to convey the recording medium a second conveying length greater than a first conveying length (S405: YES) and not for small feeds used to convey the recording medium the first conveying length (S405: NO). Since calibration is performed on the second conveying length, which is greater than the first conveying length, this method reduces a decline in precision for the conveying length of the recording medium, even when reducing the number of calibrations.