PURPOSE: To eliminate noises other than a desired picture and to improve a recognition ratio by determining conditions of an arbitrary grid point of a grid plane and values of four grid points on all sides in the vicinity of said grid point.

CONSTITUTION: A controller 7 first sets a read/write circuit 5 under readout conditions by a control signal R/W. It also controls address counters 4 and 4, successively address/designates grid points on all sides of a picture memory 2, and impresses a shift clock SCK on a five-stage shift register 6. Then a grid point (k, l) and the four-point values in its vicinity are read out and stored in the register 6. Since an output of the register 6 is given to an ROM1, an operation result is read out from the ROM1. A controller 7 puts a circuit 5 into a write condition by a control signal R/W and at the same time controls the counters 3 and 4 to address/designate the grid points (k, l) of the memory 2, thereby writing a value which is read out from the ROM1 to eliminate noises.