According to the present disclosure, an optical signal emitted from a single molecule is received to measure a central location of the single molecule while changing a phase of a structured illumination having a periodic pattern to measure a phase of a pattern in which a fluorescence intensity is periodically changed in accordance with a distance between the pattern and the single molecule while displacing the periodic pattern by a specific interval to measure the central location of the single molecule, thereby improving an accuracy of the central location of the single molecule with low photons and as a result, the resolution of the image may be enhanced.