Failure prediction of a slave device mounted with a storage device having a small storage capacity is realized in a communication system. A communication system including a drive control device, a diagnostic device, and a network is provided. The drive control device includes a control information acquisition unit for acquiring control information for controlling driving of the motor from a higher-level device, a drive control unit for controlling driving of the motor in accordance with the control information, and a transmission unit for transmitting operation result information to the network, the operation result information including information about a drive amount of the motor and a step-out of the motor. The diagnostic device includes an information acquisition unit for acquiring the operation result information and the control information, a failure prediction unit for predicting a failure of the motor based on the operation result information and the control information.