A moving image coding method that can prevent playback discontinuity without an increase in processing load during playback is provided.The moving image coding method codes a video stream that includes a first moving image and a second moving image to be overlaid on the first moving image. The moving image coding method includes: a step of determining a continuous playback section that is a group of partial sections and is subject to continuous playback, in the video stream (S5301 to S5303); a step of coding the first and second moving images in the partial sections constituting the continuous playback section, under a constraint that prevents a threshold from being changed in the continuous playback section, the threshold being used for a transparency process by a luminance key in the overlaying (S5304); and a step of generating management information including flag information which indicates that the threshold is fixed in the continuous playback section (S5305).