A radiation image data readout apparatus in which read out is performed so that a detected light quantity for each CCD element of each CCD sensor is set at a quantity below the saturation charge quantity of each CCD element, and the level of noise is reduced. The radiation energy stored on a readout-line of a stimulable phosphor sheet is estimated, and based on the estimated quantity of radiation energy, a controller sets the readout conditions, which are formed of the charge accumulation time of the readout line and the binning conditions, and readout is performed by a line sensor according to these conditions.