A passive ultrasonic tag is provided and an ultrasonic reading system is constructed more simply and records and/or reads information more reliably by using ultrasonic waves for transmission/reception. A passive ultrasonic tag according to a first embodiment of this invention includes a tag body. The tag body has two or more holes disposed extensively on a surface of the tag body. The holes have bottoms at predetermined depths from the surface of the tag body. The depths of at least two of the two or more holes are set to different values. As a result, when a three-dimensional range including surfaces and bottoms of the two holes is scanned using ultrasonic waves, the ultrasonic waves are reflected in different manners on the basis of the depths of the two holes. This allows information to be recorded and read.