JPH0616211B

PURPOSE:To decrease fluctuation in toner concn. and to permit maintenance-free control thereof by selecting the function and threshold value in a computing element at adequate value. CONSTITUTION:An incoming binary picture signal 1 is transiently stored in a frame memory 2 and the signal 1 outputted from the memory 2 enters an integrating counter 3 of black bits on one hand. The bit number of the black level in the picture signal is then integrated for one page. The signal 1 outputted from the memory 2 is inputted on the other hand to a histogram generator 4. The data on the black bit number from the counter 3 inputted to the computing element 5 and the data on the central value and standard deviation from the generator 4 are passed through the element 5 and are integrated as the black bit number subjected to correction by an image pattern in a 2nd integrating counter 6. A comparator 7 operates an operation circuit 9 for replenishing a toner and outputs a clear signal 10 to return the value of the counter 6 to 0 when the comparator 7 detects that the integrated value of the corrected black bit number exceeds a threshold value 8.