Video data transmission method for changing transmission data amounts in accordance with a transmission speed and a transmission system therefor

In a picture transmission network including networks of different transmission speeds, when a picture is sent at a bit rate which is adjusted to the transmission speed of a low-speed network, only a low-quality picture of a low bit rate is obtained in a video reception unit connected to a network of a high transmission speed. When a picture is sent at a transmission rate of a network having high transmission speed, the picture cannot be reproduced at all in a video reception unit connected to a network of a low transmission speed. To solve the problem, a video transmission unit generates at least an I picture data and a plurality of P picture data. In accordance with a request from the network, the I picture data and a predetermined number of P picture data is transmitted to the network.