In the conventional tray, the bridge for the insertion was needed on the top of the tray before interlocking the trays respectively. In new one, the interlocking is modified in such way as trays being interlocked at the side of the trays instead, which has realized the surface area reduction of the board for a tray. Furthermore, the trays are interlocked when being stacked by means of tapered shape of the trays instead of being interlocked by slots and projections. With this innovation, precise registration is no longer necessary for proper stacking of trays.