Unidirectional-aligned continuous SiCaAlON fiber-reinforced glass matrix composites have been fabricated and their light transmittance was measured. Optically transparent composites with the fiber volume fraction from 0.03 to 0.10 were fabricated by a hot-pressing method. The light transmittance of the composite perpendicular to the fiber axis in the wavelength range from 200 to 700 nm was measured, and found to decrease with the increase of the fiber volume fraction. This decrease is explained by the theory proposed by the authors (HI and YK). The major source of a light transmittance loss of the composite originates from a phase change of transmitted light in the composite.