A flow classifier having a flow CAM having plural flow entries, a search key generator for generating a search key of a format which differs by a flow condition, and a table holding a flow search result corresponding to an address of respective flow entry of CAM, in which access is made to CAM by the search key of a variable format including at least one header information item extracted from a packet header and the flow search result is obtained from the table in accordance with the address of the flow entry coinciding with the search key outputted from CAM to thereby constitute high speed formation of flow classifying by a variety of flow conditions.