An original reader capable of being downsized without requiring a large-capacity image memory. A conveying path conveys an original. Sensors are arranged on the conveying path, for detecting the conveyed original. CCD line sensors read the conveyed original. A CPU of the image reader decides the size of the conveyed original based on the results of detection by the sensors. An image memory stores image data output from the line sensors. The CPU transmits the decided original size and the stored image data to a controller. When the CPU cannot decide the original size at a predetermined timing, it transmits one of original size candidates to the controller, as a provisional original size. When the size of the original decided afterwards is different from the provisional original size, the CPU retransmits the decided size of the original.