Abstract: A transmission apparatus comprises a first data check circuit for making an error check with respect to the data transmitted through each transmission channel of a multiplexed transmission line, a selection circuit for selecting one transmission channel of the multiplexed line and outputting the data transmitted through the selected transmitting channel, a second data check circuit for making an error check with respect to the data output from the selection circuit, and a data processing section for determining the cause of an error on the basis of results of the error checks made by the first and second data check circuits. If an error is detected by the second data check circuit without being detected by the first data check circuit, the data processing section determines that the error has been caused as a result of the switching operation performed by the selection circuit.