An I/O relay interface module for mounting a plurality of relays which are adapted to be connected to individual external I/O devices to be controlled or monitored by a central controller. The I/O module comprises a plurality of relay sockets for detachably receiving the relays, respectively, and a terminal section including wire terminals internally connected to the relays through the sockets for connection between the relays and the external devices. An I/O connector is provided on the I/O module for connection with the central controller to receive therefrom control signals for energizing the relays for control of the associated devices or transmit to the central controller monitor signals from the relays as indicative of the operation of the associated devices. Also included in the I/O module is an expansion I/O connector for connection with one or more additional I/O relays provided in association with additional I/O devices such that the additional I/O devices can be also controlled or be monitored by the center controller through the I/O relay interface module. Thus, the I/O interface module can be expanded to control or monitor one or more additional I/O devices through the respective relays.