1. An anonymization device, comprising: a CPU; a memory; a singularity detector realized by the CPU, the singularity detector configured to execute a first program stored in the memory, to refer to a user information storage unit storing data items including user information, and to detect a singularity group that does not satisfy a predetermined anonymity metrics when the data items respectively corresponding to a plurality of users are grouped based on the user information; an acquired data selecting unit realized by the CPU, the acquired data selecting unit configured to execute a second program stored in the memory, and to select an acquired data item from each group based on a predetermined rule corresponding to the anonymity metrics, such that all groups satisfy the anonymity metrics when a data item is acquired from each of the groups other than the singularity group and the user information is generalized into the same value together with a data item of the singularity group; and a generalization unit realized by the CPU, the generalization unit configured to execute a third program stored in the memory, and to generate an anonymized data item by generalizing the user information of the data item of the singularity group and the acquired data items into the same value, and store the generated anonymized data items in an anonymized user information storage unit, together with a data item of each group other than the singularity group, with this data item being other than the acquired data items.