An anomaly detector includes a writing unit that writes anomaly detection data readable by an external diagnostic device to an external memory when an anomaly is detected in an on-board device. Further, the anomaly detector includes a determination unit that determines whether a failure is occurring in a memory, which is used when a processor is operated during the writing unit performs the writing. Also, the anomaly detector includes a resetting unit that resets the memory by activating a specified one of reset functions of the processor when the determination unit determines that a failure is occurring in the memory. When the determination unit determines that a failure is occurring in the memory, the writing unit writes the anomaly detection data after the memory is reset by the specified one of the reset functions.