A controller according to an embodiment includes a main processing unit and a monitoring unit. The main processing unit executes a control program. The monitoring unit monitors the main processing unit by a first monitoring method for transmitting, to the main processing unit, a transmission content corresponding to a question and for evaluating an answer to the question from the main processing unit. The main processing unit causes, when receiving an updating request of the control program, the monitoring unit to perform switching from the first monitoring method into a second monitoring method for monitoring a watchdog signal of the main processing unit.