An absorption modulator is provided. The absorption modulator includes a substrate, an insulation layer disposed on the substrate, and a waveguide having a P-I-N diode structure on the insulation layer. Absorptance of an intrinsic region in the P-I-N diode structure is varied when modulating light inputted to the waveguide. The absorption modulator obtains the improved characteristics, such as high speed, low power consumption, and small size, because it greatly reduces the cross-sectional area of the P-I-N diode structure.