We propose the fabrication of an in-line-type optical attenuator with high performance by thermally replicating carbon-dispersed polymethyl methacrylate film at the end of plastic optical fiber. When carbon with a particle size of 50 nm is employed, the dependency of the attenuation on the mode of excitation is within +/-10% even for variations in FWHM of incident modal width in the range of 10 degrees to 50 degrees . For the dependency of the attenuation on the wavelength, the value was also within +/-variations of wavelength from 400 to 900 nm.