A compressed-code generating method that is used for compressing information on characters including numerical data, sound and images, and a compressed-code expanding method that is used for restoring and expanding the compressed code generated by using the compressed-code generating method to the original information. Bit strings {y}.sub.1 and {y}.sub.2 are obtained respectively from a bit string {y} of information to be compressed. A reversible loop that exists in chaos is operated to these obtained bit strings, thereby to execute a reversible compression/expansion of the information using the chaos.