In a distributed system having a plurality of processors connected to a common transmission line, each processor comprises means for registering erroneous programs within the processor, and means for changing modes, so that a program is diagnosed on the basis of a processed result of data accepted from the transmission line and is registered in the erroneous program registration means if it is erroneous, and that the mode change means changes-over the test mode and the on-line mode by reference to the erroneous program registration means and in correspondence with the programs registered in or canceled from the registration means.