The present disclosure relates to a plastic optical fiber (POF) core diameter measuring method and a POF core diameter measuring apparatus, and a POF defect detecting method, and a POF defect detecting apparatus used therefor. Light irradiation mechanisms are provided for irradiating a side of a POF with light, with imaging mechanisms provided on the opposite side of the POF from the light irradiation mechanisms; and a data processing mechanism for processing image data on the POF acquired from the imaging mechanisms to calculate the core diameter of the POF. The ratio (D/W) of the shortest distance D to a light emission width W is in the range of 0.9 to 1.3 where W is the light emission width of the light irradiation mechanisms and D is the shortest distance between a light emission position of the light irradiation mechanisms and the side of the POF.
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