A symbol detector includes a binarization processor configured to read a symbol image in a uniaxial direction and output a first pixel value or a second pixel value obtained by binarization of a pixel value of each pixel of the symbol image, a calculator configured to calculate a second ratio of a first consecutive value and a second consecutive value obtained before and after the first consecutive value in a case where the first consecutive value and the second consecutive value are switched alternately, and a transmitter configured to output the symbol image in a case where a difference between the calculated second ratio and the first ratio is equal to or lower than a predetermined value, and stop outputting the symbol image in a case where the difference is higher than the predetermined value.