The present invention provides a test apparatus for a 64B/66B encoding process capable of precisely performing a test with a high reproducibility on a certain pattern of a 64B/66B encoder or decoder. A frame generator generates frame data in a layer higher than a physical coding sublayer of Ethernet (registered trademark) and inputs the frame data to a 64B/66B encoder such that the 64B/66B encoder performs a 64B/66B encoding process of the physical coding sublayer with respect to the frame data. A sequence pattern generator generates a certain 66B sequence pattern written in advance, and a controller writes a desired sequence pattern in the sequence pattern generator and, at the same time, controls a data selector to select one of data encoded by the 64B/66B encoder and a sequence pattern output from the sequence pattern generator and to provide the selected one to a test subject.