PURPOSE: To detect the nest level of a token string on a real time basis by inputting it to end address registers, address comparators and flag bit registers for respective nest levels.

CONSTITUTION: When the token string is inputted through a buffer memory 100, the ID codes and the body length codes of the heads of respective tokens are stored in a header holding means 120. The end addresses of the tokens are calculated by an end address calculation means 130 by using the content of the holding means 120 and the output of an address generation means 110. The calculated end addresses are held in the end address registers 140 through a switch 180 for respective nest levels. Then, the address comparators 150 compare the contents of respective registers 140 with the output of the generation means 110. The compared result is given to an encoder 170 through the flag bit holding means 160 which are reset by the compared result and shifted by the output of the holding means 120.