Object To provide a passive ultrasonic tag and an ultrasonic reading system that are constructed more simply and record and/or read information more reliably by using ultrasonic waves for transmission/reception.Solving Means A passive ultrasonic tag 100 according to a first embodiment of this invention includes a tag body 102. The tag body has two or more holes (106, 108, 110) disposed extensively on a surface 104 of the tag body. The holes have bottoms (112, 114, 116) at predetermined depths (d1, d2, d3) from the surface of the tag body (z). 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 (R1, R2, R3) on the basis of the depths of the two holes. This allows information to be recorded and read.