An image data transmitting apparatus and an image data transmitting method are provided which can distribute streaming moving images at a transfer rate in accordance with a transmission band with degradation in image quality being mitigated even at the time of congestion. A priority determining section 43 reads encoded data stored in the buffer 42 in units of VOP, and then calculates a priority for each VP in terms of VOP based on the amount of encoded data of VP and the number of MBs. A transmission control section 44 determines VPs starting from a VP of the highest priority that have a data amount up to the amount of transmittable encoded data. This determination is made based on the current state of congestion on a network 6 managed by a congestion state managing section 46 and the priorities calculated by the priority determining section 43. A VP transmitting section 45 stores the determined VPs in transmission packets for sequential transmission to a client 5.