To provide a controller, a data generation device, a control method, and a control program allowing a part of a machining program to be changed easily. A controller comprises: a storage unit that stores change designation data describing a command value in association with respective identifiers indicating at least one block among multiple blocks constituting a machining program for a machine tool; a change unit that reads the multiple blocks in the machining program sequentially, and changes the block indicated by the identifier based on the command value described in the change designation data; and an analysis unit that analyzes the block changed by the change unit, and generates execution data.