An image input-output apparatus is provided with a printing function for printing an image specified by print data from an external device and a reading function for reading an original image. The apparatus includes a printing function realizing unit, a discrimination unit, and a control unit. The printing function realizing unit operates in a first mode for realizing the printing function with a relatively small memory capacity and operates in a second mode for realizing the printing function with a relatively large memory capacity. The discrimination unit discriminates whether printing of the image specified by the print data is to be realized by the first mode or the second mode. If it is discriminated that printing is to be realized by the first mode, the control unit is adapted to allow parallel execution of the printing function and the reading function. Otherwise, parallel execution is inhibited.