To immediately know fineness, namely a denier value, only by inserting a part of a yarn into an apparatus for measuring fineness or holding the apparatus over a yarn and to avoid waste of time and labor for measuring fineness and of yarn.

The apparatus for measuring fineness is equipped with an outer diameter detection device (1) for outputting the outer diameter value of a yarn or a value corresponding to the outer diameter of a yarn, a calculation and dispaly device (2) for squaring the outer diameter value or the value corresponding to the outer diameter outputted from the outer diameter detection device, multiplying the squared value by a fixed constant for obtaining a denier value to calculate a denier value and displaying the calculated denier value. The average value of outer diameters at many points of the yarn or an average outer diameter value for certain length is used as the outer diameter value of a yarn or the value corresponding to the outer diameter.