Digital image data is divided, per frame, into "N" fields. "N/m" combinations of imperceptible codes are produced, each combination having "m" codes given by an inverse of a function giving an identification code with the "m" codes as variables. "N" and "m" are an integer of 2 or more and "m" is given by dividing "N" by an integer. The combinations are embedded into the data to embed the "m" codes of each combination into the data in "m" fields according to a rule of positional correspondence to give correlation among "m" fields in "N" fields. The data is divided into "N" fields and the codes are extracted therefrom and combined into the combinations according to the rule. An operation is conducted based on the function. It is determined that the data has not been tampered with only when the operation gives the identification code for each combination.