A controller sets the processing in accordance with an error that may occur in data. A controller for controlling a machine or a facility includes a storage unit, a diagnosis unit that diagnoses the presence of an error in data written in a memory space of the storage unit or data read from the memory space, and a processing unit that performs processing in accordance with a diagnosis result obtained by the diagnosis unit. The processing unit performs appropriate processing when an error is detected in data within a set range of the memory space in which the diagnosis unit is to be enabled.