1. A moving image playback system comprising: an image capturing apparatus that can capture, at a predetermined frame rate, a plurality of moving images under mutually different image capturing conditions in parallel in use of one image capturing element; and a playback apparatus that played back moving images captured by the image capturing apparatus, wherein the image capturing apparatus comprises: a reference condition selection unit that selects one of the plurality of image capturing conditions to be used for capturing images as a reference condition when a total of image capturing time of one frame in each image capturing condition to be used for capturing images is longer than one frame period at the predetermined frame rate; and an image capturing control unit that captures images at the predetermined frame rate under the reference condition, out of the plurality of image capturing conditions, and captures images at a frame rate lower than the predetermined frame rate under the other image capturing conditions, out of the plurality of image capturing conditions, and the playback apparatus comprises: a playback moving image selection unit that causes a user to select a playback moving image, which is a moving image to be played back, out of a plurality of moving images captured by the image capturing apparatus; a detection unit that detects a motion between frames of a moving image captured under the reference condition when the image capturing condition of the playback moving image is an image capturing condition other than the reference condition; and a conversion unit that converts the frame rate of the playback moving image into the predetermined frame rate by generating an interpolation frame for interpolating between frames of the playback moving image based on the motion detected by the detection unit when the image capturing condition of the playback moving image is an image capturing condition other than the reference condition.