A printing apparatus comprising: a reading means for reading a document to generate image data; a first storage means for storing the generated image data; a printing means for executing printing based on the image data read from the first storage means; and a second storage means for storing image data read from the first storage means, wherein speed at which image data of the document read by the reading means is stored in the first storage means is higher than speed at which the image data is read out from the first storage means for printing, characterized in that the printing apparatus is able to execute a copy process where the reading means reads the document and the printing means executes printing based on image data generated by reading the document, and the printing apparatus further comprises: a control means for (i) performing control such that the image data that is stored in the first storage means and is not subjected to printing by the printing means is overwritten by image data of a document newly read by the reading means and (ii) performing control such that image data for one page is transferred from the first storage means to the second storage means after the image data for one page is stored in the first storage means; and a determination means for determining whether there is print target image data in the first storage means, wherein in a case where the determination means determines that there is the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on the print target image data that is read from the first storage means without being read from the second storage means, and in a case where the determination means determines that there is not the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on image data that corresponds to the print target image data and is read from the second storage means. a reading means for reading a document to generate image data; a first storage means for storing the generated image data; a printing means for executing printing based on the image data read from the first storage means; and a second storage means for storing image data read from the first storage means, wherein speed at which image data of the document read by the reading means is stored in the first storage means is higher than speed at which the image data is read out from the first storage means for printing, characterized in that the printing apparatus is able to execute a copy process where the reading means reads the document and the printing means executes printing based on image data generated by reading the document, and the printing apparatus further comprises: a control means for (i) performing control such that the image data that is stored in the first storage means and is not subjected to printing by the printing means is overwritten by image data of a document newly read by the reading means and (ii) performing control such that image data for one page is transferred from the first storage means to the second storage means after the image data for one page is stored in the first storage means; and a determination means for determining whether there is print target image data in the first storage means, wherein in a case where the determination means determines that there is the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on the print target image data that is read from the first storage means without being read from the second storage means, and in a case where the determination means determines that there is not the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on image data that corresponds to the print target image data and is read from the second storage means. a control means for (i) performing control such that the image data that is stored in the first storage means and is not subjected to printing by the printing means is overwritten by image data of a document newly read by the reading means and (ii) performing control such that image data for one page is transferred from the first storage means to the second storage means after the image data for one page is stored in the first storage means; and a determination means for determining whether there is print target image data in the first storage means, wherein in a case where the determination means determines that there is the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on the print target image data that is read from the first storage means without being read from the second storage means, and in a case where the determination means determines that there is not the print target image data in the first storage means and where the printing apparatus normally executes the copy process, the printing means executes printing based on image data that corresponds to the print target image data and is read from the second storage means.