Provided is an effective index processing method for a key corresponding to characteristics of a key series. The index processing method includes: holding a key tendency for representing the characteristics of the key series and a node split ratio, corresponding to the key tendency, for representing a key split ratio at the time of node split; and switching the node split ratio of an index (162) based on the key tendency. The key tendency/distribution is determined based on characteristic information of data input by a user or monitoring information acquired through monitoring of the data.