A controller includes a microcomputer that operates in a normal mode or in a low power mode and communicates with an external device, a monitor circuit that monitors an operation state of the microcomputer based on a monitor signal output from the microcomputer, and a start circuit that controls a drive of the monitor circuit. Communication signals exchanged between the microcomputer and the external device include a dominant state and a recessive state, and the start circuit monitors the communication signals. When the communication signals in the low power mode of the microcomputer include the dominant state, the start circuit puts the monitor circuit in a monitoring state. When no monitor signal is input from the microcomputer to the monitor circuit that is operating in the monitoring state, the monitor circuit determines that an abnormality has occurred in the microcomputer.