A quantization unit for quantizing orthogonal transform coefficients to calculate a coefficient level: calculates the number of value information bits of a significant coefficient level of an orthogonal transform coefficient to be processed in an image block; monitors coefficient levels output from the quantization unit in the image block from beginning to the N-th prior to the target in transmission order, and updates a state variable indicating whether or not at least one of monitored coefficient levels is significant; calculates the number of position information bits of the significant coefficient level of the orthogonal transform coefficient to be processed, based on a position of the orthogonal transform coefficient to be processed and the state variable; and sets the significant coefficient level to 0 in the case where the number of value information bits and the number of position information bits satisfy a predetermined condition.