An apparatus is provided for detecting transmittance of a trench. The trench is located on an infrared-transmittable material, which can be a wafer. The wafer is obtained after a ditching process. An image of the wafer is fetched. The contrast of the image is greatly enhanced. The contrast-enhanced image is used for automated analysis of the transmittance of the trench. Accuracy of detecting the transmittance is improved. Hence, the present invention uses a simple structure to detect transmittance defects of the trench for ensuring goodness of the wafer.