To effectively execute a safe and reliable washing work for a shellfish on shipboat.

There is provided an apparatus for washing a cultured shellfish includes the endless belt 21 which configures a conveying conveyor 20 and a holding conveyor 30 by forming a slit channel 9 passing a crossover culture string 70 from an inlet side 6 to an outlet side 7 at a front side of a body 3 of the washing apparatus 1 at a height nearly equal to a conveying surface of the conveying conveyor 20 in a horizontal direction. The endless belt 21 is entirely formed in a net-like structure by respectively alternately combining a plurality of connection pieces formed to have identical shapes. The apparatus includes an adjustment part 60 of a fixed tension for uniformly stretching the endless belt of the holding conveyor 30 and an adjustment part 65 of movable tension for preventing deflection of the holding conveyor 30 during washing.