The invention concerns an automatic sorting apparatus for sorting out pieces under process, such as fish and shellfish or industrial products, into different size groups. In the prior art automatic sorting apparatus, pieces under process are readily caught in sorting holes, making it difficult to obtain smooth sorting. The invention permits smooth sorting. An automatic sorting apparatus to this end includes an endless conveyor having sorting holes and capable of excursion, an inner receiver disposed inside the endless conveyor for receiving pieces under process falling from the sorting holes, and an outer receiver disposed outside and below a U-turn portion of the endless conveyor for receiving pieces under process falling off the U-turn portion. Another automatic sorting apparatus includes two or more endless conveyors having different excursion diameters and disposed one inside another at an interval. The sorting holes of each endless conveyor are smaller than those of the next outer endless conveyor. An inner receiver is disposed inside the innermost endless conveyor for receiving pieces under process falling through the sorting holes of that endless conveyor. An outer conveyor is disposed outside and below a U-turn portion of each endless conveyor for receiving pieces under process falling through the sorting holes of that endless conveyor.