PURPOSE: To use a network as the whole of a system even when a data repeater receive an abnormally long frame.

CONSTITUTION: With respect to the frame processing method of a data repeater 10 which is connected to the transmission line constituting each network and processes and repeats data frames transmitted between communications terminal stations of the network, the length of the received data frame is detected, and if it is longer than a prescribed length by the abnormality of communications terminal stations, the transmission line, or the like, a part of the repeating function(functions of a reception part 11, a repeating control part 12, and a transmission part 13) of the repeater is stopped to stop repeating of the data frame.