A data transmitter is provided. The data transmitter is configured to transmit reception data and copy data of the reception data to the outside via respective separate ports. The data transmitter includes an identification information attaching unit configured to attach, to the reception data, identification information identifying the reception data, a reception data copying unit configured to copy the reception data with the identification information attached thereto, a data calculation unit configured to perform the same calculation operation on the reception data with the identification data attached thereto and the copy data copied by the reception copying unit, and an identity determiner unit configured to determine whether the reception data with the identification data attached thereto is identical to the copy data, based on the identification data and the calculation results of the data calculation operation.