A scrambled transmission system for transmitting a digital data containing variable length coding conforming to a MPEG standard includes a scramble key generator for generating a scramble key at a timing of each GOP head. A GOP head detector detects each GOP head and causes the scramble key generator to refresh the scramble key. A seed generator generates a scramble seed based on said scramble key at a timing of each slice head. A slice head detector detects each slice head and causes the seed generator to refresh the scramble seed. A random number generator scrambles the digital data based on the scramble seed. A multiplexer embeds the scramble key in the extension.sub.-- and.sub.-- user.sub.-- data of the scrambled data.