An efficient encoding/decoding apparatus of this invention comprises orthogonal transform element (10) for orthogonally transforming a digital signal obtained by allowing an analog signal to undergo analog/digital conversion, quantizer (11) for quantizing the digital signal which has undergone orthogonal transform processing, inverse quantizer (21) for inverse-quantizing a digital signal which is not yet caused to undergo inverse orthogonal transform processing, and inverse orthogonal transform element (20) for inverse-orthogonally transforming the orthogonally transformed digital signal, wherein rounding in even number direction or rounding in odd number direction is used in at least one of the orthogonal transform processing and the inverse orthogonal transform processing. Thus, accumulation of rounding errors at the time of, e.g., such a direct digital dubbing to repeatedly encode/decode digital signals such as video signals, etc. is prevented, thus making it possible to reduce picture deterioration of multi-generation characteristic.