The invention in one embodiment is an apparatus. The method is particularly applicable to represent information so as to be perceivable by a hearing-impaired person wherein a plurality of images, when displayed, one after another on a display device, represent information perceivable by the hearing-impaired person via sign language. According to such a constitution, the signal light power per one wavelength included in the output light of each of the optical amplifying devices can be kept constant, irrespectively of the number of wavelengths. The storage unit stores a halftone table in which an LPI higher than a normal LPI is set.