PURPOSE: To read out other items of this data, by inputting a data item in a data storage device storing a series of relating data.

CONSTITUTION: A memory for data storage is split into, e.g., four blocks A, B, C, D and they respectively store items of commodity number, commodity name, price, and number of inventory. Each block is written in the data corresponding to the block and the stored address of other items relating to the data. If the number of inventory of "C-77" is desired to be informed, a block A including the "C-77", the "C-77" itself and a block D are inputted with a keyboard. Thus, the "C-77" is searched from the block A, the content d9 of the address a12 at the pst stage is searched out and the content of the address d9, that is, inventory number "2,500" for the "C- 77" is picked out.