1. An image sensing apparatus comprising: an inputting unit configured to input sensed image data; a reducing unit configured to reduce the sensed image data into reduced image data of a first density; a detecting unit configured to detect an instruction for sensing a still image during sensing of a moving image; a storing control unit configured to control a memory to store image data of a second density higher than the first density based on the sensed image data inputted after detection of the instruction for sensing the still image; and a processing unit configured to, in a case where the detecting unit detects the instruction for sensing a still image during sensing of a moving image, process a first moving image frame by using first reduced image data of the first density during a first time period, process a second moving image frame next to the first moving image frame by using second reduced image data of the first density during a second time period after the first time period, divide a still image frame into a plurality of parts by using the image data of the second density based on the sensed image data inputted after detection of the instruction for sensing the still image, process a first part of the divided still image frame during a third time period between the first and second time periods, process a third moving image frame next to the second moving image frame during a fourth time period after the second time period, and process a second part of the divided still image frame during a time period between the second time period and the fourth time period, wherein during the third time period, the processing unit does not process the first or second moving image frames regardless of whether or not the detecting unit detects the instruction for sensing the still image during sensing of the moving image.