PURPOSE: To extract an image data having a value of optional order from plural image data without sorting them.

CONSTITUTION: This device is provided with N stages of examining circuits 100-400 corresponding to respective bits of N-bit image data and an order number (m) is inputted to the examining circuit 100 of the uppermost stage. The examining circuit of an (i)th stage (i is 1 to N) finds new examination signals XP2-XT2 showing which of M image data is candidate data on the basis of the value of the (i)th bit from the most significant digit bit of each image data as to image data specified as candidate data by examination signals XP3-XT3 supplied from the precedent stage. Thus, examination signals are found in order for every stage and image data having an (m)th value is specified with examination signals XP-XT obtained at the examining circuit 400 of the lowermost stage.