A tag device causes a second calculator to read a confidential value from a confidential value memory and to apply a second function F2 which disturbs a relationship between elements of a definition domain and a mapping thereof to generate tag output information. The tag device delivers the tag output information to a backend apparatus. Subsequently, a first calculator reads out at least part of elements of the confidential value from the confidential value memory, and applies a first function F1, an inverse image of which is difficult to obtain, and a result of such calculation is used to update a confidential value in the confidential value memory by overwriting.