The carbon-modified TiO2 were synthesized through microwave carbonization of ethanol by using a domestic microwave oven. This process enabled to form the carbonaceous compounds of the surface of TiO2 and created several new mid-gap bands into the original bandgap within few minutes operation. The sample showed a remarkable visible-light absorption even at the wavelength of around 800 nm. The promotion of photocatalytic activity under visible and ultraviolet (UV) light irradiation were also confirmed by the I-3(-) formation in KI aqueous solution. The I-3(-) formation rate of carbon-modified TiO2 per unit mass under visible light is almost 25 times higher than that of pure TiO2. The mid-gap optical absorption mechanisms were investigated through analysis of absorption edges. It is revealed that surface state change against microwave-treatment time results in different mid-gap optical absorption processes. [DOI: 10.1143/JJAP.47.8456]