Two or more microphones receive acoustic signals and generate audio signals that are processed to determine what portion of the audio signals result from incoherence between the audio signals andor audio-signal sources having propagation speeds different from the acoustic signals. The reflection surface of the rotary polygonal mirror and a scanning position of a peripheral surface of a photosensitive drum are set in an afocal and conjugate relationship by first and second cylinder mirrors. 13 times the average thickness of the product portion of the thin molded product and a molten metal velocity at the product gate is set to not less than 30 ms and, moreover, a solid fraction Fs of the semi-molten metal M and a grain size D of the solid phase of the semi-molten metal M are set so as to define the relationship FsD1500.