A pattern presence/absence evaluation value generation unit (41) determines the presence or absence of a pattern by comparing detection data (d3) and correction data (d44). A correction data updating unit (42) outputs correction data (d42) updated by weighting and adding the detection data (d3) and the correction data (d44) by increasing the weighting of the correction data (d44) when an image pattern is present and increasing the weighting of the detection data (d3) when the image pattern is absent according to a pattern presence/absence evaluation value (d41) from the pattern presence/absence evaluation value generation unit (41). A correction data subtraction unit (43) subtracts the correction data (d42) from the detection data (d3) and outputs output detection data (d4). Consequently, it becomes possible to reduce the influence of detection signal level variations of detection signals generated within the read period of an object to be detected and accurately read the pattern.