PURPOSE: To improve the compressing ratio by extracting a difference between two picture elements of a multi-value picture data and forecasting this difference to widen a referenced region thereby improving the hit rate of forecast.

CONSTITUTION: When an input picture signal 20 is set to a register 10, the content stored so far in the register 10 is moved to a register 17. Output signals 21, 22 of the registers 10 and 17 are applied to an ROM18 as a means obtaining the density change between adjacent picture elemens. The difference between the two output signals is extracted at the ROM and a value taking 16 as a normal and a change amount signal 31 are outputted. This signal is applied to a first- in first-out register 12 via a register 19 and also to a register 19, and three reference signals (difference signals) are applied to an ROM14. A forecast value 36 is outputted from the ROM14, the change amount of the density with coding picture element difference signals A∼X is calculated by an ROM15 and a forecast error signal 37 is outputted.