PURPOSE: To improve the degree of recognition by obtaining (n) units of data train from a picture data train showing the brightness of a prescribed viewfield where an object to be detected exists to normalize those data trains and calculating the difference between said data trains and the normalized picture data train related to the object of detection set previously.

CONSTITUTION: An ITV camera 10 photographs an object to be detected (round rod) 1 and gives an A/D conversion 11 to the bright/darkness signal into 16 gradations to give them to a shift register 12. The register 12 containes 10 stages equal to the number of picture elements existing in the crossing direction of the rod 1. The picture data added to these 10 stages are defined as a train A and normalized 13 into A'. This train A' is applied to an arithmetic circuit 14. While a data train B having the most standard brightness in the crossing direction of the rod 1 is set to a picture data setter 15. This train B is normalized 16 to the data train B', and the train B' is supplied to the circuit 14. The circuit 14 obtains the sum total SK' of the absolute value of the difference between the components of the trains A' and B'. Such an operation is executed every time the new picture data is applied to the register 12 to obtain the minimum value. Thus the presence of the rod 1 can be recognized with high accuracy.