Translucent α-SiAlON and β-Si3N4 sintered ceramics were fabricated by employing different amounts of AlN–MgO additives. Relatively high transmittance in both visible and near infrared regions was obtained at large additive content. High visible transmittances but low near infrared transmittances were obtained at small additive content. The starting Si3N4 powder and sintered bodies contained unpaired electrons which are derived from N vacancies. The decrease of near infrared transmittance is due to the interaction between these unpaired electrons (polaron) and light wave. When the additive content was large, oxygen diffused into Si3N4. Combination of oxygen with unpaired electrons seems to decrease the concentration of unpaired electron in sintered body, resulting in high transmittance in near infrared region.