The conversion factor in a CCD device is determined by the photon transfer technique in a region where pure shot noise is not dominating, i.e., where noise of the readout electronics must be taken into account. The method is based on the measurement of the noise standard deviation, dependent on the intensity of the signal and the fifing of a simple model incorporating shot noise and signal-independent noise. The result is a linear equation system. From its solution the conversion factor and the standard deviation of the signal-independent noise of the device are determined. The mean number of generated charges is determined from the measured mean signal by means of the estimated conversion factor.