According to an embodiment, a generation method includes: setting one of multiple first nodes as a second node; selecting (M-1) (M is an integer of two or more) fourth nodes from among one or more third nodes on the basis of a directed graph; and writing, to a first storage area, an information piece related to each of the second node and the (M-1) fourth nodes. The multiple first nodes are included in the directed graph and correspond to vectors in a search range. Each of the one or more third nodes is an out-neighbor node of the second node in the multiple first nodes. The information piece is an element related to one first node of the index information corresponding to the directed graph. The information piece includes a vector value of the one first node and IDs of all out-neighbor nodes of the one first node.