An IP address look-up device is for looking up an IP address of an IP packet in order to forward the IP packet. The device includes a ternary CAM including pairs of data string and mask string, with the data string having a prefix of an IP address and the mask string representing a length of the prefix, and having routing entries stored in physical segments, respectively, without regard to order of prefix length, the ternary CAM functioning to compare a search key as a destination address of the IP packet to be forwarded, with the routing entries, set a match line of a matching routing entry, and output a mask string of the matching routing entry; and a priority encoder functioning to compare prefix lengths of mask strings outputted in correspondence to set match lines when at least one match line is set in the ternary CAM, determine, as an LPM entry, a routing entry corresponding to a longest mask string, and output a physical address of the LPM entry on the basis of a match line of the LPM entry.