A level detecting part detects image data levels for the respective output directions of the first half and the second half to be used for the linearity correction, which image data levels correspond to image data around a connection point between the first half and the second half and are obtained from respective combinations for the respective output directions of the first half and the second half. A calculating part calculates linearity correction values or correction value calculating parameters from the detected image data levels for any one of the first half and the second half; and a defect detection part detecting a defect in the image data levels thus detected around the connection point are provided.