An image data processing method to capture still images while capturing motion images is disclosed. The method includes stopping a motion image capturing according to a first instruction to generate a first motion image data with a first resolution, thereafter, carrying out live view at a predetermined time; outputting the image data in the predetermined time and generating a first temporal motion data with a second resolution; capturing a still image data according to a second instruction after the predetermined time; transforming the first temporal motion data and the still image data and generating a second motion image data by combining the transformed data with the first motion image data. The method and device according to the present invention can be used in a camera to capture still images while capturing motion images.