In a system in which a host computer and MFP are connected via a communication medium such as a network or the like, upon transmitting image data stored in a box of the MFP to the host computer, the transmission data size (file size) is displayed before transmission. The user can transmit image data with a desired file size. When the amount of transmission data is large, the load on resources such as a memory, CPU, and the like of a device as a destination of transmission can be reduced.