1. A moving-picture decoder for decoding a coded moving-picture signal that has been coded by a discrete cosine transform with motion compensation and includes full-resolution motion vectors, comprising: a resolution-converting IDCT processor performing an inverse discrete cosine transform with resolution conversion on said coded moving-picture signal, thereby generating a first picture signal with reduced resolution; a resolution-converting motion compensator using said full-resolution motion vectors and a reduced-resolution reference picture signal to generate a second picture signal with reduced resolution; an adder coupled to said resolution-converting IDCT processor and said resolution-converting motion compensator, adding said first picture signal and said second picture signal to generate an output picture signal having reduced resolution; and a memory coupled to said adder and said resolution-converting motion compensator, storing said output picture signal for use as said reference picture signal.