The present invention provides an image data processing method of recording image data coded with the MPEG (Moving Picture Experts Group) technique to a recording medium. For transition of the bit occupancy in a VBV buffer to a target value, the initial value of a bit occupancy in a VBV buffer is calculated on the basis of auxiliary data read from a recording medium, a comparison is made between the target and initial values of the bit occupancy, and the number of bits for assignment to each GOP of to-be-coded image data correspondingly to the result of comparison.