Crops are required to be processed in sanitary manner for lunchbox production in industries. Particularly, foreign object detection is crucial to guarantee consumer's health. Actually, foreign objects are mainly detected by human's eyes. For the automation, image processing is an example, however, is not effective for hidden objects. We propose a method of foreign object detection with ultrasonic reflection analysis. In our experiment, we selected four crops (lettuce, spinach, perilla and komatsuna) for masking material, and a coin for occluded objects. The coin were attached on the backside of the masking material. We recorded reflected wave moving the material linearly. As the result, reflection intensity doubled by the existence of occuluded object.