PURPOSE: To exactly obtain a blanking level by detecting the blanking level of input digital video signals for every specified period and correcting the value of video signals at an encoding or decoding part.

CONSTITUTION: For a Y signal 104 digitized at an A/D converter 3a, the blanking level is calculated for every fixed period at a blanking level detecting circuit 12a. This calculated blanking level is compared with a black level value calculated by the encoding/decoding device or encoding/decoding method, and a differential value 202 is calculated. This differential value 202 is outputted from the blanking level detecting circuit 12a. The Y signal 104 inputted to an input signal value correcting circuit 4a is impressed to an encoding circuit 5a as a Y signal 107 which signal value is corrected by subtracting the differential value 202 at a subtracter 9a.