Abstract: An image data transmission apparatus, which may be used as a facsimile apparatus, includes an image data reader which is detachably connected to the main body. The reader can read image data from an original fed past the reader, or it can be detached and used to scan an original. A device exists within the apparatus, which detects the mounting or unmounting of the reader, and if it is detected to be unmounted and not being used, power will be shut off. Further, a device exists, within the apparatus, which, when the reader is detected as mounted, will print read data. Also, when the reader is detected as unmounted, read data will be stored in memory. Thus, a speedy and power efficient image data transmission apparatus has been created.