To scramble the I/P pictures of an inputted picture signal without changing an MPEG system by providing a vertical and random offset by the unit of a slice.

A scrambling device 12 replaces one side of a picture in a frame memory 11 for an invalidated macro block of a 16×16 pixel consisting of gray, etc. A key generator 13 generates a key randomly selected from the sequence of 0 to 3 and allocate which of the upper/lower and right/left sides of each frame the invalidated macro block is replaced by this, next, the area of an invalidated pixel is moved to the right or lower tip of the picture. Thereby, horizontal or vertical random variability is provided for each frame to scramble. An MPEG encoder 14 processes the scrambled picture and generates I/P/B pictures in accordance with the sequence of GOP. At this time a selector 15 is controlled to scramble only the I/P pictures.