According to an embodiment, a data processor includes a storage unit that stores a set of character strings that is a set of character string elements of which magnitude correlation is uniquely identifiable; a data converter that encrypts numerical data included in data to be managed, generates an index value corresponding to the numerical data using the character string elements included in the set of character strings, and generates converted data including the encrypted numerical data and the index value; a first transmitting unit transmits the converted data to a server; a query expression converter that converts a condition part including a numerical value of a query expression into a condition part including the character string elements to generate a converted query expression; a second transmitting unit that transmits the converted query expression to the server; and a receiving unit that receives, from the server, a result of query.