PURPOSE: To obtain a coding image quality which does not deteriorate the coding image quality even when information is lost in a communication line and is stable, even when the information quantity of an input image signal is suddenly increased.

CONSTITUTION: Since an error coding control part 12 performs the independent control to make constant the image quality, the quantizing step size at an error quantizing part 11 does not become large. The increased quantizing error is again quantized and outputted by the error quantizing part 11, an output code quantity from the error quantizing part 11 is increased, the increased portion works so as to compensate the reduction of the image quality at the output of a quantizing part 10, and therefore, as a whole, the coding image quality can be constantly kept. When an image coder is applied for the image transmission using a transmission line so that the losing of the information can occur, the information coded and outputted by the image coder is thought to be gradated to two steps of the output from the quantizing part 10 and the output from the error quantizing part 11, the output of the quantizing part 10 is transmitted with priority, the losing of the information occurs and even then, the influence is not given to the prediction between frames in which only the output side of the error quantizing part 11 is selectively lost (the losing of the information does not effect the local decoding output).