We have developed a controller for the beam monitor system of XFEL/SPring-8. The control items are the electronics of a beam position monitor (BPM) and a current transformer (CT), the actuator of a screen monitor (SCM) and the stepper motors of a SCM lens system and a beam collimator. To operate such complicated devices, we designed a control system based on a programmable logic controller (PLC). The PLC enables us to implement complex commands easily. For the communication between the PLC and other equipments, we employed FL-net (a protocol to communicate with computers) and DeviceNet (a connection with peripheral devices). Since FL-net is strongly supported by MADOCA, the official framework of SPring-8, we can easily integrate the PLC into the upper level system. DeviceNet is used in the electronics of the BPM and the CT and in the motor controllers of the SCM and the collimator. We newly developed a DeviceNet-based motor controller to satisfy our complicated requirements. Another multi-wire cable is equipped to manipulate the SCM actuator and so on. We tested the beam monitor controller at the SCSS test accelerator and confirmed that it worked well.