A programmable electronic controller in which one central arithmetic processing unit and a plurality of input devices and output devices are connected by means of a parallel bus, the controller being basically configured to activate a self-diagnostic function and a diagnostic test of the input devices and the output devices with an instruction from a microprocessor of the central arithmetic processing unit; and to judge the result with the microprocessor of the central arithmetic processing unit, by using the microprocessor installed in the central arithmetic processing unit also as a processor for tests (diagnostic tests) of the self-diagnostic function of the input devices and output devices and conducting tests of the self-diagnostic function of the plurality of input devices and output devices with the central arithmetic processing unit.