To provide a method for updating an operation program, a system for updating an operation program, and a main device, for enhancing operability in updating the operation program such as firmware, when a plurality of projection devices are used in such a manner that one of the devices is used as the main device while the other of the devices is used as a slave device for the purpose of achieving a stacking function or the like.

The main device 1 accepts update data (S1); temporarily stores the update data (S2); and sends the update data to the slave device 2 (S4). The slave device 2 receives the update data (S5); temporarily records the update data received (S6); and sends a reception completion signal to the main device 1 (S7). Upon receiving the reception completion signal, the main device 1 sends an update command signal to the slave device 2 (S9); and updates the operation program according to the temporarily recorded update data (S10). Upon receiving the update command signal, the slave device 2 updates the operation program according to the temporarily recorded update data (S12).