PURPOSE: To simplify a correcting circuit by obtaining a difference of correction quantity between reference dots provided in every scanning line and obtaining a correction quantity in every scanning line on a basis of a correction quantity, which is obtained by dividing equally this difference, and correction quantities corresponding to reference dots.

CONSTITUTION: Correction quantities corresponding to convergence adjusting dots are stored in a one-frame memory 6. A correction quantity A of the second rows of dots is read out from the memory 6 and is applied to a multiplexer 17. A correction quantity of the first row is read out from a 1H register 7 and is applied to a multiplexer 18. A subtractor 19 obtains a difference between correction quantities A and B and divides equally this difference in accordance with the number of horizontal raster scanning lines, and the correction quantity obtained by this equally dividing the difference is added to the correction quantity B successively to output the correction quantity from an LPF13.